Hm Well, my first thought was lymph nodes which we have in our pits. But when a lymph node is swollen there, it rarely hurts. So, then I wonder about little cysts, infected hair follicles, etc. I would say that the best thing to do is to see your doctor and go when you've got one. I do not really think it's swollen lymph nodes, to be honest but one of the other scenarios or one I'm not aware of. But a doctor is best to evaluate. Do you have any behind your ears? Down your neck? Just wondering.
It seems like you have cyclic mastalgia (periodic breast pain) caused by fibrocystic breasts. Breast cancer rarely presents with pain, and the lymph nodes would stay swollen. The lumps and discomfort are related to hormonal changes due to your menstrual cycle. It's pretty common.
If it isn't this, then it's simply lymph node swelling due to menstruation which happens, too.
Go to the doctor and they could test thyroid levels, and other hormone levels to see if something is wrong!