Chest muscle pain?

I have what feels like muscle pain on my left side of my chest. I believe this is due to me carrying my 10 month old son around alot. He is very clingy and likes to be held all the time. He weighs 18 lbs and gets heavy after a while. I tend to only carry him on my left side. Could my pain be due to me using my muscles on my left side the most. I have anxiety disorder and I always worry about every little ache and pain I have. I think it's just strained muscles... what do you think??

Forgot to say... the pain is not constant, it only comes if I move a certain way and then it's gone. It feels like a strained muscle. I figured it was from carrying baby on that one side only.

Do you primarily feel it during those activities and it goes aways with rest? Are there certain positions you assume that cause the pain (such as lying a certain way), but then it relieve when you get out of that position? If so, you pain is most likely mechanical in nature (muscular, or from the neck, etc.). If you get pain that seems to occur for no reason and stays that way no matter what position or motion you are doing, it could be a medical cause...especially if it is accompained by other unexplained symptoms. If you experience the later, best to follow up for medical care.