I would not risk taking an appetite suppressant while breastfeeding. I'm sure you know this, but breastfeeding requires just an additional 500 calories or so, so if you are consuming more than that additional amount, it will be a little harder to lose the extra weight.
Is the weather nice enough to get out? Wearing your baby or pushing him in a stroller would amp up your workouts a little if you're not already doing that.
Are you sure you're eating enough calories? If you don't, your body will hold onto fat, because there isn't enough coming in.
What types of food are you including in your strict diet?
Are you eating the same calories and doing the same exercises every day? If so, they may no longer be a challenge and your body is so used to them, it just expects that. Try changing things up - both calories and exercise.