What Causes Spotting With Cramps During The Menstruation?

I had an iud fitted 4 weeks ago and am due to go back to have it checked but cannot go until the clinic opens tomorrow. My period appeared to start 4 days ago- light bleeding and it still hasn't become full flow despite severe cramping pain similar to when the iud was fitted. I can feel the threads and they feel the same length as when the iud was first fitted. How do they check it at the clinic- just the threads in the same way I do? I'm very confused, have never had this much pain without full menstrual flow before.