You are most likely feeling your abdominal aorta; especially if you are slender. Just about everything should be able to feel this forceful pulsation. Most of the time you feel it while lying on your back. Just do a google search for pulse in stomach. You'll find dozens of others with that same question.
I have the exact same problem. My mother freaked out a second ago and told me to call my doctor and get some kind of tests done on the stomach. She said whatever it is its in my stomach. And in the heat while tanning it gets 10 times stronger. The doctor should be calling me back any second so hopefully I'll get more answers. I'll let you know, but try and get your stomach checked... fast!