Hi, Stinging and burning sensations in the nipples can be due to various causes; however, I want you to be assured that mostly its normal considering your age. If you are about to get your periods in the next 7 to 10 days then changes in hormonal levels can make your breast and nipple tender thereby leading to your complaints. The next reason can be constant friction between your nipples and the shirt/coat that you wore combined with the cold weather. This should have made your skin dry thereby leading to pain. You have also mentioned that you borrowed a bralette from your friend although this is inadvisable given the circumstances I can understand your situation. You need to first understand that bra s work by providing a support to your breast against gravity. So when you take it off, there is sudden force of gravity coming into play this can intensify the pain and burning already present. The final thing is that it can be a sign of pregnancy as well. Given your age and no history of sexual intercourse this is unlikely. I would suggest you that apply some warm compresses, moisturize your breast and nipples, wear a comfy bra that you feel comfortable. It might be hard to find appropriate sized bra as your breasts might still be changing in size. If it prolongs for more than 7 days consult with a gynecologist or your pcp.