What Causes Brown Menses For 3 Weeks?

I have had a tubligation so.I don't believe I am pregnant. For 3 weeks I had a "brown" period but it was not enough to need anything more than.a pantie liner. 4 days ago I started what seemed to be my normal period but lighter than normal. I usually only have cramps for.the first 24 hours. This time I am having severe cramps and it hasn't gone away nor have I stopped bleeding