hi .. welcome to GW ... you got it last w/end.. and you have already fertilized it twice??? one might suggest you are on the verge of loving this thing to death ... and you water it when it needs water ... insert finger.. that medium looks rather peaty to need watering every other day ... but it is really up to your finger to figure that out .... and when i bring home a plant.. i NEVER place it out in full sun .. since i have no idea if it is acclimated for full sun .. i would give it a few days in very bright shade.. to get it hardened off... and then start moving it toward full sun ... [it looks like a greenhouse plant.. and any glass between the sun and a plant.. in theory.. is not full direct sun ..] .. and with the plant getting rather large.. and the pot getting smaller.. i might avoid full sun altogether ... the pot just might not be holding enough water for all day in the sun .... put it in bright shade .. water it properly.. dont fert it for at least a month.. and then very diluted.. and just enjoy it... once it goes to seed.. there wont be anymore flowers.. unless you take that one off .... good luck.. and good day ken ps: dont make presumptions just on the name.. because.. in this case.. a sunflower.. probably should be in mother earth.. rather than a pot ... and that is where problems start ...

Ants can sometimes mean you have an aphid problem. They like to eat the aphid secretions and will protect the aphids. But I do not remember aphids being a big problem with sunflowers. Sunflowers need full sun and that could also be a reason for the yellowing of the lower leaves. Annual sunflowers are a weed in my area. A picture would help to see how the plant is doing by the size of the leaves. Also, where you are growing them and in what climate would help. In almost all of the mainland US sunflowers would not be getting enough sunlight or heat to be growing this time of year. I have not grown sunflowers in a long time but they did not seem to be a problem, as long as they were planted after the soil warmed up.

More than likely that is from the larva of one of many insects, or maybe even the adults. Are there wee green colored thingys on the underside of the leaves? Do you see any insects on the plant at all?

That was my firs thought. I have had them before. No lines, leaves looks like this in 24 hours. Nothing then this? Seems to quick, but it does look like beet leaf miner pics I found.