Hello, I Have A Question. I Had Unprotected Sex On

Hello, I have a question. I had unprotected sex on Saturday morning. Monday morning I got up, and had spotting and cramps. It went from bright red, to brown. It completely stopped on Tuesday, but the cramps didn't. I still have those. Ever since Monday, I have been extremely tired and thirsty. I feel like I'm dehydrated. And I'm even nauseous. What could this be?