Missed period?

I have been taking the same bc pills for over 6 years without any problems. AF usually lasts 2-3 days and is very light, always regular and can pinpoint exact day that I get it each month as it comes on the fourth day of the week of pills without the hormones. Well, this month I should have had it May 1st-3rd. Never came. I haven't missed any pills so it is not that. On the 3rd, after peeing, I wiped and there was like this string of brown blood on the TP and then that was it. Nothing further. Well I have taken 4 HPT with the last one being today may8th and all were negative and taken with FMU. My husband and I are not TTC right now...planning on it about 6 months from now. Just wondering what everyone thinks and if this has happened to anyone. Pregnant somehow? Just missed a period this month? Who knows.