I've had the Mirena for 4 year now. My periods stopped after the ...

I've had the Mirena for 4 year now. My periods stopped after the first month. From then on I have had light spotting for only 3 days. That changed two months ago. First of all my period was 18 days late for March but it finally came on and lasted for 10 days, it was very heavy with clots. I was also having some pregnancy symptoms before my period came on such as nausea,constipation,sore breast,and fatigue. I took some pregnancy test but they were all negative. For April the my period was the same like it was in March, but I am feeling little flutters in the bottom of my stomach and having a lot of cramping on my left side. Yesterday I tried to feel my string but couldn't. What's going on? I need for help!!