Okay So I Started My Period August 7th And It

Okay so I started my period August 7th and it stopped on the 13th (this is normal length for me) I had unprotected sex on the 16th and the 19th. I started light bleeding on the night of the 19th. I've been bleeding for 4 days and it's kinda heavy. I don't know why I'm bleeding again but I'm kind of scared. I'm 16 and overweight (I don't know if this matters)