Sore breast on Nexplanon?

Good morning! I got my Nexplanon inserted on September 1st, I got a light, light period starting on September 21st (right on my schedule). I've had the Nexplanon and understand that your menstral cycle changes (mine got lighter). But, im pretty much over with the period and now my boobs are sore? Could I be pregnant? Just a new side effect? I waited two weeks before sex and we always still use condoms. Just never had sore breast before!