Bleed 2 weeks after period

I came off the pill 6 mths ago (after being on it for 15 years). I had a v. light period 1 mth later, then a normal period a month after that in May. Then I had nothing until 2 weeks ago when I had quick a heavy period that lasted 5 days.
Then today (2 weeks later) I noticed a bit of bleeding when I wiped (brownish with light pink). We are trying for a baby, but I have done a test which came back negative. Any ideas what this could be?
I have had some mild cramping on and off.