I went through the same thing with DS a few times..

First time he was teething but had no teeth, just clamping down and drawing my nipple through his gums - Yowch! I watched for when he was going to do and said a firm NO, and popped him on the floor, (cue screaming baby) he only ever did it a few times.learned VERY quick.
second timehe was older and had all his teeth, CHOMP..testing my reaction, maybe I wasnt paying enough attention?
anyway same thing but more explanation, "no biting mummy, if you bite mummy no boobies" on the floor, walked away, DS howled, never did it again.

Good luck