I have chosen to keep pumping too, instead of using cows milk. My 18 mos son is up a lot at night nursing. We have moved to not nurse in the 4 hours before I have to get up for work, rest helps make milk too.
In my experience it seems to be that I have to eat a lot of calories, (enough to slightly gain) to keep my supply up. If I start to loose any weight at all my supply tanks. Remember, if you aren't getting REM sleep, then your body is keeping your metabolism up all night.