I took 3 pregnancy tests, all negative. I've been getting my period for the last 4 months but I can feel a pulse in my stomach for about a month now, is it possible that I am pregnant?

OK, let's sum up what you have told us, shall we?
You took a pregnancy test, and it was negative.
You took another pregnancy test, and it was also negative.
You took a third pregnancy test. Result? Negative.
You took a sonogram that was negative.
You took a second sonogram, also negative.
You have regularly been menstruating for the past five months.
On the other hand, you feel a pulse in your stomach. Is it possible that you are pregnant? No. Not a chance.
When the embryo’s heart starts beating, it is plainly visible on ultrasound. At this time, it is about 1–2 mm across. How did you feel it, exactly? You would not feel a 1–2 mm diameter heart if it were beating in your hand, and you were looking right at it.
What was the rate of the pulse you felt in your stomach? At five months, the heart rate is about 140 beats per minute, having earlier peaked at about 170; it will remain at 140 for the rest of the pregnancy.
If you can plainly feel a heartbeat of about 72 beats per minute, get yourself evaluated for an Abdominal Aortic Aneurysm.