I know it's easier said than done, but there's no need to jump to conclusions.
I had brown discharge twice in the first tri (a few weeks apart) and everything was fine. Just keep an eye on it. If it doesn't get worse or if you don't have any cramps it should be fine. I even had a bit of pink spotting and everything turned out fine. I did get checked out, it was very stressful, but I got a bonus ultrasound.
Have you had sex recently? It could be that your cervix is irritated. I'm pretty sure that's what happened to me.
How far along were you?
The first time I was around 5 and a half weeks, the second time around 10 and a half.
The first time it lasted a few days and I did have some cramping, so I went to the clinic to get my levels checked, then saw my doctor who advised me to go to the hospital... Long story short I got the runaround, and at the hospital they were SURE I was miscarrying, the ER doc (who was actually a resident so I don't think he really knew what he was looking at) told me my cervix was open, and they thought my levels weren't rising fast enough (but by that point they were in the 30,000s so even I knew that they weren't supposed to be doubling)... Anyway, I got a transvaginal ultrasound to rule out ectopic because I was also having one-sided pain. It confirmed a viable intrauterine pregnancy.
It's normal for there to be brown discharge 24 hours after sex... In my case both times it was more like 48 hours afterwards. I think sometimes your cervix takes a little while to heal after something like that.
And I'm pretty sure sex was the reason because we hardly had sex at all in the first tri so the link was pretty clear.