Sore nipples but not sore boobs

Monay

So I’ve been on bc pills since May of this year. I am having unprotected sex with my boyfriend. I haven’t had a period in almost 50 days. My nipples are sore but not my boobs. I spotted a little bit this month for 3 days and then that’s it. I took a pregnancy test last month and it came back negative. I am cramping here and there sometimes. Is this the birth control that’s doing this?