Answer: It's tough to say what is causing your pepper leaves to curl, but here are some possible causes that match the symptoms you describe:
It could be Pepper or Tobacco/Tomato Mosaic Virus. Symptoms include stunted plants, distorted leaves and fruits and reduced yields. Check your peppers for aphids and spider mites. They are the common carriers of this disease.
It could be damage due to the application of herbicides (Did you spray anything on other non-edible parts of your garden?).
It could be weather related. Leaves often curl and plants quit growing due to heavy rains or when excessive amounts of moisture stays in the soil for prolonged periods of time. The upper leaves are usually the first to curl. Peppers like water, but they don't do will when over-watered. Keep their soil a bit on the dry side without letting it ever dry out completely.
It could be due to extreme temperatures. Cool temperatures (day or night), especially when peppers are first transplanted, can stunt their growth. Peppers like it hot, but prolonged periods of extreme heat can cause stress and result in permanent damage.
At this point I would just wait and watch. It may very well be weather related and the plant will recover. Hope so!
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