Sam - this could be stress, indigestion, or a sprain but before you think of heart attacks (which I think you are) it could be a pulled muscle but more often than not its something really very innocent. Obviously you are concerned or you would not post here but I am going to ask you to nip in to the surgery and ask to see a nurse and talk to her. If she feels anything is really wrong she will get a doctor to nip up to see you. Get some peace of mind as nobody here is qualified to diagnose your pain but it sounds to me like nothing is life threatening and the sooner that someone qualified tells you the self same thing then the sooner you will relax. Savvy?