Help?(!)

I am a junior in high school and I have never ever taken a sex ed class (my school never offered them). I don't know anything about sex but my boyfriend has practically taught me the basics (which I am SO grateful for).

We have frequent sex but rarely with a condom, is this bad? He doesn't come in me so i thought that it would be okay? But today, after we had sex, I found a weird reddish-brownish discharge on my panties although I'm in my fertile window. What does this mean? Could I have any advice?

Thank you ☺️