I have a pulsing on the lower left side of my stomach. it doesn't hurt, just sometimes it feels uncomfortable.. is this normal? what can this be?

Probably normal: If you push on your abdomen it is bound to be uncomfortable. Would stop and see if it is still uncomfortable and if so see your MD

Aorta: The only thing that pulses there (that you could feel) is your aorta. The incidence of aneurysm in a 30 year old female is close to zero but it's conceivable that the pulsation is being transmitted through an overlying structure so an ultrasound is indicated. If you're very thin, it's likely entirely normal.

Intestines?: It's quite possible that you are feeling the movement of your large intestine digesting which can cause the experience of what you are describing. That is my best guess. This is harmless unless you are in severe pain and/or are having difficulty with your bowel movements in which case you should seek medical attention immediately.