Seems like there's a heartbeat in my stomach?

That is your aorta: The aorta is the largest blood vessel in the body and in the abdomen, runs in front of your spine. A slender person may feel this normally, particularly if excited, or blood pressure is high. In a normal or overweight body type, you may want to get this checked to make sure you don't have an enlarged aorta (aneurysm). A simple ultrasound is an easy way to check it if your doctor is concerned.