Hi qxeenb, whenever you have an anxious thought about you heart or stomach recognize the thought, then put it out of your mind as soon as possible. When you recognize the thought as an anxious thought tell yourself, 'this is just a thought, there actually is nothing wrong with my heart or stomach', and move on to something else.
I think it is important to let these kind of thoughts go quickly, don't entertain the 'what if' something is wrong for any length of time, just put it out of your head so that you don't get into the habit of thinking and brooding.
Take care! :)