leaking boobs

I'm 28 weeks pregnant and on 4 separate occasions I've felt my boobs leak. I don't see any colostrum or milk actually come out but I feel the insane unquenchable thirst and the tingly sensation of the letdown. Is this normal in pregnancy? I've asked my doctor and he said it is as long as the dischArge isn't green. I haven't told him about the thirst or that I feel physically sick a few moments before this happens. (Belly gets very tight and uncomfortable).