Did you transplant it recently? If you did, it might be going through shock. It probably is also running out of energy for new healthier grow. I went through similar problem with my other berry. I kept them in the platic container from Wal-mart, they had been growing slow and the new leaves were not vibrant. I recently put them in ground, and mix the soil with some "Peat Moss", the new leaves are now looks better, I guess they get more new energy/nutrient now. Raspberry needs acidic soil, get some Peat Moss into your planting area, and get some soil acidifier powder from your local nursery, mix it with water then spray the plant and ground around the plant. It should improve within 2 weeks.

Overwatered maybe? With to cold temperatures or too much wind?

New growth isn't a clear indication that the plant is growing. True growth is measured by a plant's increase in dry weight; so, if a plant is shedding a leaf and growing a leaf, it's roughly in a static state, which is not a good place for a plant to be. I'm not necessarily saying the leaf ISN'T being shed as a result of normal senescence, only that new growth might not indicate an o/a increase in mass. There is no better fertilizer program than one you can control to the degree you know exactly what nutrients your plant is getting, how much of each, and when nutrients will be available for uptake. There is an ideal level of nutrients in the soil and an ideal fractional amount of each in comparison to each and all of the others. Most plants use about 10X as much N as P, and about 3/5 as much K as N. The rest of the nutrients also have an ideal level determined by a function of N levels and the overall level of total dissolved solids (TDS) in the soil solution. Adding tonics, miracle concoctions, or supplements targeting an increase in 1 or 2 nutrients w/o good reason or a strong likelihood what you are doing is going to correct a deficiency, is far more likely to result in limitations than benefits. Al

It is possible you have Early Blight. Mine do the same thing. They will be fine until the first full day of high heat and humidity then it just explodes. The first thing to do is get all the bottom leaves trimmed off - I've read that it is fine to take everything off at least the first foot of your plants and even up to 18". Get them in the trash then wash your shears and gloves with bleach. This will help stop the spread. Copper fungicide or Daconil sprays, or Epsom Salt drenches are usual recommendations. I usually start spraying mine about 2 weeks after planting. Good luck with yours.