What Causes Brown Spotting Instead Of Regular Periods?

Im 25 .I had sex using withdrawal method two or three months ago. Then I had a strange period wich was lighter than usual, with milder cramps and spotting wich went on and off. First day brownish in color, then stopped for a day and then more with a red tint. I though I was preggo becouse before the spotting my period was late so I took a hcg and it was neg. After a week or less I took another and still neg. So I thought I m not preg. and left the situation alone. But after a month same thing with my periods. It been three months now. I was 8 days late and the brown spotting. Sorry for being this long. What do you think?