I have been going to the gym for 12 weeks now. i'm circuit training, watching what i eat, and breastfeeding. no weight loss...any idea why?

Gaining muscle: If you are truly strength training: observing good technique, using muscles in full range of movement, exercising muscles to exhaustion, and increasing resistance accordingly, them you should be seeing a change in your body proportions. You should be experiencing an increase in you energy level and noticing a change in the way your clothes fit. These are more important than "weight loss.".

Fewer calories?: It sounds like you've been working hard and you are probably making more progress than you realize so don't give up! Circuit training is a great weight loss tool. I would focus on your calories "in" now. If you are not losing weight, chances are your calories (no matter how healthy) are still too high for you to lose weight. Go to www.myfitnesspal.com for a great support and tracking site.