irregular period. From very strong to very light almost nothing

I used to have very strong periods. Specially after my baby was born. I got the IUD right after she was born and for two years my period was regular. First 3 days were the strongest and then it was getting less. Usually had it for 5 to 6 day. But last month when i had it it was really light. Almost nothing. I felt a lot of cramping(really painful) and for 7 days it was that one day i was bleeding then it stopped and then again a little and so on. And it looks like this month is the same. I thought i could be pregnant but test came out negative. Should i worry and run to my doctor. Please help me . Thank you