What Causes Wet Spots And Yellowish White Discharge ?

Hi, I am 19 weeks pregnant and have had wet spots on my underwear for over a month. It has gotten progressively heavier but the underwear is not completely soaked. It looks like a sweat patch underneath the armpit. I went to the ER almost two weeks ago and was assured that everything was fine and that it wasn't amniotic fluid. I called me ob today and she said I should schedule a sonogram just to be safe. I have one scheduled in two days. Sometimes the discharge feels like a pee sensation but it does not smell like urine. The last 4 or 5 days the clear wet discharge is accompanied by yellowish white discharge. I am scared that it may be amniotic fluid. Could it be amniotic fluid or is this normal?