I have a lot of unexplained bruising that all doctors I have spoken to (GP, GI, and rheumatologist) have just brushed off. It's generally polka-dot formations on my legs that I know for a fact are not coming from any sort of trauma.
I know low platelets are associated with polka-dot type bruising. If you're on any medication or have any vitamin deficiencies, those might be a likely culprit. It's probably nothing to worry about, just make sure you bring it up with your doctor to be sure.