What Causes Pink Vaginal Discharge?

Im 23 weeks pregnant and Ive been having on and off pink discharges since november (usually it comes out right after i pee). my Dr did labs for UTI or BV and even took an U/S of my cervix. but everything is normal. Im worried because i had a m/c in June 2014 and got pregnant after 2 months.I dont want to be a bothersome to my Dr everytime i have a pink d/c,but its scaring me too much.