almost non-existant period...what's going on?

Ok...so normally my periods last 5 - 6 days and are normal to heavy. This has been the way - like clockwork - since i started menstruating when i was 13. I'm only 27. I am on a low-dose pill, but it's the same one I've been on for over a year. My boyfriend and I never use condoms (we live together). Anyways - last month my period concerned me cuz it was only about 3 days long - but it WAS there so I stopped thinkin' about it. And now - this past month - the day it was due - last saturday - i wouldn't even have noticed it was there unless I wiped. Really nothing on sunday or monday either. There was no need to even wear a pad or anything such as cuz i really had no flow at all. I took a HPT yesterday and it was negative. Last night after he and i were "together", I went to the bathroom, wiped and there was a little bit of blood, not much but a little.

Should I be concernced? Should I take another test? I took the test yesterday, Tuesday - and it was negative - and my normal start day was to have been saturday.

concerned in indy