I Am Convinced I Could Be Pregnant, But We Use

I am convinced I could be pregnant, but we use condoms every time we have sex. I am 17 years old and I just recently got my period about 4 days early. It started as a normal period, with red blood but the next day was more of a dark color with brown mixed in. The third day there was barely any bleeding, as well as the fourth day. It is the fifth day now and there was only a little brown discharge. I got very lightheaded and nauseous today at school and had to go home early, but other than that I haven't been experiencing any other early pregnant symptoms. I have acid reflux and also have been under a lot of stress so I don't know if those are other factors. I just don't want to be pregnant. I took one test on the third day or the short "period" and it was negative.