What Could Cause Very Heavy Periods?

I had a very heavy period last month, had to use tap on and pad together and change every 2-3 hour for 4-5 day then normal bleeding for a further 3 days. It was a week later than normal but I have been trying to conceive. The following month I had spotting just when I went to toilet instead of my period. Now I should be in the middle of my cycle and I keep getting brown stringy discharge and brown spotting. Had a negative pregnancy test should I be worried?