pulsing sensation in abdomen

I started getting a strong pulsing sensation in the middle of my abdomen, towards the left side.  I'm not experiencing any pain.  I feel it mostly when I'm sitting or lying down.  I believe I'm too young for it to be an abdominal aortic aneurysm (I'm 25).  Could it be something stomach or intestinal related?  I have an appointment with my doctor in a couple days but would like to get some ideas as to what could be causing it.

[jessNabbey]

Hi
I too experience a strong pulsing sensation right in the middle of my chest, just below my rib cage. It is strange because I can actually see it pulsating when I lie down. I am also worried because I have a strange feeling sometimes that I can't catch my breath, or take a deep breath which seems to be a symptom of an aneurysm. Let me know how you go at the doctors.