What Causes Missed Periods With Pink And Brown Vaginal Discharge?

Hello, I have now missed 2 periods, I have done 2 hpt, both negative and had blood test done at the doctors also negative, I had one day where I had a light pink watery discharge and then the following day a dark brown discharge, this was 5 days before my second period was due???? (my first period was due on 20th June and my second on 14th July, my last period was 27th May), I have been trying to conceive for the last 14 months