help after breast feeding..

I need help. I stopped breastfeeding for personal reasons when my son was 5 weeks old. In short- the pain associated was far worse than I hAd ever imagined. I then got mastitis, and thrush. Anyway. Fast forward, it has been about three weeks since I stopped. My breasts are smaller and closer to prepregnancy size. They’re still firm, but I’m thinking that’s residual milk.

My worry is that i sometimes get pain behind my boobs. Yesterday it was on the left side, strong and stinging and I thought “shit, it’s chest pain!” And I wanted to go to ER. But it went away. Today i got the same pain but behind my right boob. Does this sound normal? I don’t have heart problems. I really didn’t want to spend $200 on an ER copay. I just need some advice. Thanks guys