Brown fluid and blood spotting

I'm really scared because I had sex in a hot tub without a condom (he didn't finish inside me) on August 2nd and I'm supposed to get my period around August 22nd. Today, August 20th, there was a brown stain on my underwear, and it looked like it could be blood but it was dark brown. When I went to the bathroom, there wasn't really any blood, but a piece of tissue came out of my vagina and it was brown. I decided to put a tampon in and when I took it out a few hours later, there was a lot of brown fluid and some blood, but not a lot. Could I be pregnant? I took a pregnancy test that came out negative but I think it was too soon when I did it because it was on the 15th. PLEASE HELP!