I think I just had a miscarriage....?

I've been on my period for a week now and it's been extremly heavy this whole time. Just now, I stood up and I felt a gush. I ran to the bathroom and a huge clot fell into the toilet. (The size of a golfball) I looked at it and it didn't look like a clot. It looked like tissue. I'm scared to death. I'm 20 years old. Someone give me advice...