Does breastfeeding prevent some women from losing weight?

For the past two months I have been exercising 5x a week, taking a 1.5 mile walk 5x a week, and watching what I eat, mostly eating protein high, low carb, very little added sugar except what is naturally in fruit. I would guesstimate I eat under 2,000 calories a day... my Apple Watch estimates I burn about 400 calories from exercise and I am burning at least another 400 from breastfeeding. BUT I CANNOT LOSE A SINGLE POUND. I seriously have not lost one pound in 2 months of this consistency. I cannot figure out why. I can’t restrict calories anymore or I will lose my breastmilk supply and practically tank my metabolism. Is it because I’m breastfeeding that I cannot lose weight? I’m not really massively overweight anyway, I’m 5’7” and weigh 174lbs. I just want to lose about 20lbs. But I can’t seem to! Help?