My 8 month old has 2 little what seem to be bruises on the back of her back, could it be Spina Bifida?

No. Spina bifida is obvious. The occult form is not but doesn't present as a "bruise". A flat bruise like skin discoloration around the base of the spine or buttocks is most likely a birthmark called a Mongolian patch. It is relatively common and has no significance more than its tendency to cause concerns.