Could be a type of period where you haven't ovulated for some reason. There are lots of reasons why this could happen and most of them are innocent ones. Is this the first time this has happened? Have you any other symptoms (pain on intercourse, pain in pelvis generally, any discharge, spotting etc?) If it is a one off, I'd be inclined to wait until the next month when you got your AF and see what it's like. If OTOH it's been going on for a while, I'd pop along to the GP and ask for some tests.
I had this in May which surprised me as AF is normally regular and fairly heavy. I went to the dr's to have my hormone levels checked and 2 blood tests revealed that I hadn't ovulated that month. Next month had a normal period and went on to conceive DB2!
HTH. Let me know how it goes.