Scarleteen

Before I got my period I noticed some brown gooey blood on my underwear. As I was reading I read about implantational bleeding or something like that. I was getting worried because I was already late on my period and was having a pregnancy scare because I had sex about two weeks ago. (With a condom) after about 3 days of having it I got my period. Why did this happen? What was the brown stuff after all?

Periods vary a lot, especially over many years. And sometimes that means menses starting or ending brownish and thick.

Unless you're not up to date with your STI screenings, I don't see any cause for concern. But if you have it and want to look into it, a visit to your sexual healthcare provider is your next step.