What Could Cause Missed Period, Low HCG Levels, And Brown Discharge With Back Pain?

Hi My last period was from18-22 Sept my next circle was supposed to be on the 15 October and I missed that. On the 18th October I went for Quantative Beta HCG test and it showed me 1 of which I think it s negative then on the 25th October I saw a brown discharge when I was wiping after urinating an till today 26 October but it is only when I wipe my self it s not a lot. What might be the cause and I have back pains also