Help!

So, my period is 13 days late. I took a pt and unfortunately, it was negative. I just feel like something is wrong. I have been having some weird cramping lately. Lately it feels like sore muscles and in the beginning, it felt like period cramps. I've been feeling very nauseous. One two occasions, I have noticed I had brown discharge after using the restroom and wiping. Could I still be pregnant or is it something else?