I've Always Been A Tampon User But The Last Year

I've always been a tampon user but the last year or so I've noticed that when I use a tampon my flow will stop and I get the most awful cramps! When this first started I would double over in Pain but when I'd go to the bathroom I would feel the urge to push and I would end up pushing out really large dark clots and then the pain would stop. Now this time it has stopped, the major cramps are here but despite my efforts I can't get anything to push out. What does this mean?