it might be what they call " old blood " as you haven had a period for a few years if your really worried get it checked out

I get that sometimes (and I have very irregular periods.) I wouldn't worry about it if I were you, but for future reference, you can get advice on things like that from NHS Direct. I'm sure you can google for the number. It's a freephone service, manned by nurses. Useful for when you could do with a bit of advice but are worried about wasting your doctor's time.

it's just old blood i am sure.