PAINFUL period and unexplained large lump?

So I had my period on time, I've always been on time with my periods. They're never painful either, I get slight cramping and the occasional headache etc. However, this period was a complete different story and I'm concerned. I had HORRIBLE cramps to where I could not even move, I just had to lay in bed. When I went to the bathroom I changed my tampon and I saw a HUGE lump. I mean large, it was about the size of a golf ball (but not round like one, it was more flat) and it was hard, a little squishy but more hard. After that came out my cramps stopped. I have NO IDEA what happened. Has this ever happened to anyone else? I took two pregnancy tests before my period started just to be safe because I had some unexplained cramping (slight cramping though) before my period was due. Both tests came back negative, I had no signs of pregnancy but the first thing that popped into my head was a miscarriage? I hope it wasn't but I have no idea what it could be, I've never heard of this happening or had it happen. I've looked online and can't find anything to explain this. I also have my annual exam next week so I plan on asking about this but I also wanted to see if anyone else had ever had this happen. Thank you!