I'm 23 years old and in good physical condition. I do not drink or smoke.
The last time I had sex was in the month of may, I had unprotected sex with my fiancé. I got my period on time for the next 3 months, I had some heavy flow days.
Well, I recently found out that my fiancé was having sex with a married woman while he was "with" me. My period was due on the 1st of August, m over a week late and I have been spotting for a couple of days now! I get a brown discharge with slight traces of blood. I'm worried if it might be an infection. I'm pretty sure I'm not pregnant, but I'm really worried of an infection.
I haven't been sexually active since may and that was the oly time I've had unprotected sex!
Please help!

This brown discharge IS your period. The color brown is just dried blood. No, you are not pregnant and no, you don't sound like you have any infection that would affect your period like this. You just are experiencing a light period this month. Don't freak out! This is normal.