You know, you really sound like you worry too much. Believe me, that can be one of the most unhealthy things you can ever do. Didn't you say you were really young? Listening to your body is important and all, but be careful not to let your imagination run away with you in times like these.

I'm not a female, but here's my guess: sometime very recently, you used your pectorial muscles for the first time in a while. Your right side was fine because you are (probably) right handed, but your left side was so underused and underdeveloped that the muscles there had to work like heck to keep up, and now they are sore. The area you described is one of the prime regions for muscle soreness after physical strain. Does that sound like it could be it?

Tell your husband/boyfriend to be a little more gentle in bed with them