Is there anything that would cause my spleen to feel like it's beating?

Yes: Although very rare, an aneurysm of the artery that feeds the spleen can give one the impression that the spleen may be "beating". This can usually be diagnosed by an ultrasound of the abdomen. Keep in mind that the spleen sits right below the diaphragm on the left side and the heart is just above the diaphragm on the left, so you may simply be feeling your heart beating.

Unknown: Most of us cannot feel our spleens posterior, under rib cage. If ones spleen is big enough to feel that is an issue by itself. In thin individuals, the aortic pulsation is easily felt. If not thin, concern would be aortic enlargement.