random bleeding?

okay so wtf... I just had my period about 2 weeks ago and I'm extremely regular. Me and my boyfriend haven't had sex in over a month and I just randomly started bleeding? Idk if it's a lot bc I have yet to do anything about it but sit on the toilet... my boobs have been sore ever since my last period which never happens and my last period was a full on heavy period too so it wasn't like implantation or anything.... I'm not on birth control either.. wtf is going on?? I'm only 17 and I'm nervous like I'm so regular and everything's the same every month except now.....