What Causes Tail End Brown Bleeding At The End Of Periods?

HI i have tail end brown bleeding at the end of my period, i had a scan yesterday which showed no endo, it also showed both ovaries had ovulated. My bloods are normal, ie progesterone and estrogen. What could this be, i have always had this, but am currently attending a fertility doctor, he cannot find anything wrong except tail end brown bleeding. I have had two previous miscarriages and also suffer from Ulcerative colitis which is in remission now. thank you