L.R.
Please see a doctor as soon as possible! The symptoms you describe could come from ANY of a wide range of problems -- do not assume it's arthritis or muscle strain or anything else. It could be nothing or it could be serious; we aren't doctors.
And as for being sure it's not your hear -- women tend to wait too late to respond to heart attack symptoms. Our symptons are not as clear as most men's. If you are also experiencing any fatigue or shortness of breath, then yes, this could even be your heart. But there is no way for us to diagnose you. Please see a doctor.