Either spider mites or a lack of proper watering...The top leaves look wilted which is not a good sign(

MIke

I don't see any yellow leaves, the fallen ones that you show look to be healthy. You need to slip your tree out of the container and examine the roots. Insure that the growth medium's drainage is good so that the root zone oxygen supply remains sufficient after you water the tree. One of the distress symptoms shown by a discarded citrus leaf is that the leaf bade only falls without the stem attached. - Silica

Mike i got no bug issues damaging plant.

Silica. There is signs of yellowing if you look at 1st photo all the leaves that have dropped have these yellowing marks near blade. Process just started some dropped leaves look light green and leaf blades also drop.

Can it be that i left those many little fruits on tree too long + new growth drained lots of tree energy on them, forgetting to maintain old leaves?

No, little fruits would not cause that if your tree was healthy...It's natural for your tree to bare fruit..The leaves look wilted and as Silica say's, Id make sure the the mix is good and make sure the roots are getting completely watered right...

Roots are fine, i`ve changed the soil in end of January. When i water the plant i check with moisture meter. Problem occurred after blooming when tree set lots of little fruits. One thing i haven`t done when plant was active fertilizing it like in summer i kept winter routine, every 1-1,5 month.

Any more suggestions what can be wrong?

That looks to me like a pot that doesn't allow much air exchange. The wilting, for me, is an issue with the roots. And the pot or the soil seems a likely culprit.

Getting sun and lots of light? Temperatures? Any sudden change in climate, location, light?

Before i mentioned that leaf blades also drop, i wasn`t accurate. They do drop but, first leave drop and some leaf blades hand for 2 days. Many blades are still hanging. And tree still keeping them not giving up.

This soil mix is holding moisture. In winter i water every 2-4 weeks while there is no sun so much. In summer watering is every 3-7 days. Mix contains Soil for citrus +perlite+ pine bark. I need a mix that holds moisture because can`t not be home often in summer. so can`t water every day or two. Recently 2 sessions i was using same mix with some perlite without pine bark. Then watering was in winter 1-1,5 month, summer 7-12 days.

There was sudden light change i`ve recently used 2 bolts: 1 LED 12w Red&Blue 3:1 ratio. 2 CFL w 6500k. Month ago i`ve changed lighting system from that i mentioned to T5 39Wx2 6500K.

Temperatures in room are from 68F to 74F, and when i ventilate the room temperatures drops to 62F. Plant is located in Climate zone 5-6. it is facing east windows, getting 11 hours of sun light (there is still not much direct sunlight) + turning on artificial light for 1 hour now to maintain 12 hours of light.

Well, the change in lighting may be a factor, but usually leaves yellow (much more than yours) and then drop off.

I would keep the mix moist but somewhat on the drier side and see how it goes. I still think it's a soil/root issue, but if you say there isn't waterlogging, I'll take your word for it.

If it's a change in conditions issue, then the plant will recover when it adjusts.

Could be the lighting this T5 is greater than 2 bulbs i had. Today only 1 leaf droped, blade is hanging. I noticed that most leaves turning light green with those little yellowing signs.

Oh and i noticed looking after plant every day. There are still some blooms left coming but its something strange they are developing so slowly. Or even stopped

I could check the roots for safety. But tell me if its safe to irritate roots so often? Cause i have changed soil about 2 months ago.

I usually take something long and pointy and displace a chunk of soil six inches deep to see the moisture situation down there.

I would say that your tree's medium has good enough drainage with the ingredient mixture you used. I notice that your tree's container is setting in a saucer. Do you let the tree remain in the saucer when it is full of water that drained from the container? If you have a soil thermometer you might check the container's medium to see what the temperature is, especially if you use cold tap water. Citrus roots have no active function at temperatures of 55.4-F. and very little function at temperature above but close to 55.4-F. The amount of water that the soil container is not the actual problem, The actual problem for over wet soils is that in poorly draining soils their is not enough root zone oxygen. - Silica

I use always room temperature rain water. Not letting water to stay in the plate. Today i have took citrus out of pot and looked at the roots, in my opinion they are fine (but im not expert). Took photos of the roots. Soil at surface was abit wet, medium moisture not wet, ant upper was kinda dry.

What do you think?

1st side

Other side

Closer in to the roots

Yep, they look absolutely fine. Must be the light then. It'll recover.

The roots look OK, but the root system as shown in your picture looks to be very dry. - Silica

It were near watering time, did not watered to look at the roots. After plant back to container gave a good watering. And today seems that plant leaves reacting to light. Hoping that leaf greening,yellowing and droping stops :)