Good for you for nursing full-time! That takes real patience, dedication, and perseverance. It was one of the most rewarding things I ever did.
Not sure if this applies or not, but if you're eating "way below [your] recommended calorie intake," that could be your problem. If you eat too few calories, essentially (and this is high unscientific), you won't burn but will store. Some people call this starvation mode. This could be especially true if you're nursing. You need extra calories to nurse. Check with your lactation consultant about your RDI, don't just guess.
"The grass ain't greener, the wine ain't sweeter, either side of the hill" The Grateful Dead