If your a smaller person [thin, shorts, etc.], you may randomly miss your period. Pregnancy tests don't lie. If it says you're not, esp after four, then you aren't. What it sounds like is you missed one and you have a very light period. The brown/red is blood. It's just mostly dead. Dead blood during your period is normal.

The pain may simply be that this peticular period is giving you worse cramps than you're used to. Try taking a couple Advils when stuff like this happens. It worked for me, and many people I know.

It's normal.