pulsation in stomach please help

i'm 18 years old and weigh about 145
for the past 2 weeks about i've had a pulse in my abdomen area i can feel it sometimes but other times i have to push on my stomach to feel it actually i push on my belly button area.  I dont know what it is.  i also feel a little pain sometimes.

[Cleveland Clinic]

Given your age, I would suspect that there is nothing wrong with you and that this most likely is your descending abdominal aorta that you are feeling pulsating.  One way to check is to count your radial pulse and then put your fingers on your belly to feel the belly pulse and see if its similar to the radial pulse.

[Pum]

This is normal for slim people.