There are any number of reasons why you could be experiencing blood coming from your belly button after a tummy tuck. There could be a vessel which has not closed off and is leaking. This would be unusual after 8 days. There could be a small collection of blood (hematoma) which is communicating with your belly button. Several other possibilities come to mind but the short answer is you must see your surgeon and have he or she diagnose and treat the problem.
Very often, putting a small piece of gauze into the incision around the belly button can wick out the blood that is oozing, speed your recovery and decrease the chances of infection.
With reference to your question as to whether bleeding of this nature is normal, the answer is, it is not normal but it certainly not that unusual and if no other symptoms are present, this should be easily treatable and curable.