You're breasts are expanding and its gonna hurt. Even if no milk is expressing yet, suction can help (using a breast pump gently or your creativity ;)... relaxing helps it also can help to use warm compresses or a hot water bottle. This is going to get worse... sorry no better answer than that. Good luck!
Your breasts are getting ready for the baby. They will get bigger especially your nipples.
That is really common in pregnancy and the only thing you can do is just handle the pain. I had the same problem from when I first found out I was pregnant until I was about 32 weeks and the pain just now going away and I'm now 35 weeks. But there isn't really nothing you can do about the pain.