I started to notice like a pain or idk maybe a soreness under my armit near my breasts

hi I had sex three weeks ago on January 2. Than on January 6 I got my period. At least I think and hope it was. It was like my normal period like normal color and had like little clots and stuff. During the time I was very stressed and scared. We had sex but it was protected and he said he pulled out but who knows. it was both our first times. Anyway my period was normal except it was lighter but still decent flow and it lasted three days it usually last four or maybe five. I wasn't sleeping or eating and I heard stress can affect your period. It is now January 23. Everything has been normal and I've been ok. Today I started to notice like a pain or idk maybe a soreness under my armit near my boob i guess but I don't think it is my boob but it's close to it. It's inly in my right side? Is this a symptom or? Please help me I'm scared.