As a rare side effect, Humira can cause lowered platelet counts, resulting in easy bruising. Not to be alarmist, but I would let your doctor know about this right away, to make sure that the correct tests are ordered.
Hope this turns out to be something that's quick and easy to deal with!
While on the Humira I started bruising alot. (This was not the case while on Enbrel) I got brusies from professional medium-touch massages, even. My platelets were checked and still normal, so the doctor isn't concerned. Now I am on Remicade instead and it seems to be about the same situation. Wish you well!