Sudden irregular periods, and there's no way I'm pregnant

I feel rather awkward asking this on the internet, but money and time currently don't permit me going to the doctor.

Usually, my period is anywhere between 30 and 35 days, and it's a fairly normal flow; heavy the first couple days and gradually tapering off, with mild to moderate cramping the first day.

This month, on the 35th day, I wasn't even sure I'd started, as there was only the small amount of spotting that you'd expect to find on the last day. It stayed the same for the entire week, with some days not so much as a drop of blood on the pad until late evening. Everytime I went to the bathroom, there was no blood in the toilet and on the paper was, once again, the sort of light brown spotting you'd expect on the last day. Now, this bothered me a little, but I thought nothing of it until I started my period again after only 19 days, and it was once again a very light flow. There was no cramping either time.

Now, I still live at home, and I'm fairly well synced up with my mom and younger sister, and I'm thinking this time may have been caused by mom starting; but I usually start the day she finishes, never at the same time. I'm not on any kind of birth control, and I'm still a virgin so there's no way it's a pregnancy scare. The only thing different about my diet is that I started taking hair skin and nail vitamins about two months ago.

Another thing that worries me, is that for months now I've been finding an odd bluish-black stain in my panties. It's not smelly, and it looks like the sort of thing you'd see if you scribbled with a black marker and ran water over it so that the ink bled. This also started before the vitamins. I thought at first it was color from the sheets I was using at the time, but the sheets have been changed and it's still there. Nor is this in anyway a laundry problem, since I wear nothing but white underwear and it's always in with the bleach load.