It is a common problem with dieters. Back in the days of cave people, it was a good survival mechanism for your body to go into a low idle when food sources were scarce. For dieters in this day and age, it is a curse. Generally, exercise is a good method for countering that effect, but perhaps your walking is not intense enough to do that. Also, add weight lifting two times a week to your routine.
Muscles burn more calories than fat, plus a muscular body has a higher metabolism.
The increase in calorie intake along with the exercise will raise you metabolism and therefore help you to lose weight. It is not safe or healthy to lose weight rapidly, especially while nursing. You do not want to risk a reduced milk production, or a depletion of your bone and muscle tissue for the sake of getting back to your prepregnancy weight. In the scheme of things nursing your baby, at a less than ideal weight, is really best for the health of your baby and for your long-term health. A quick glance at that sleeping babe of yours will reassure you that it is worth it.