Maybe you could try changing things up a bit... a program like Insanity or P90X that pushes you a bit and does moves that your body may not be as used to.
Also, are you happy with your size, but just not your weight? You're a small person in general, but maybe pregnancy changed your body a bit so your pre-pregnancy weight may not be as appropriate as it used to be. I know it's changed mine... but I've never been a very small person either.
Insanity worked well for me when I lost after my first pregnancy, so maybe it will work for you as well

I agree with the other post about possibly changing your routine! Sometimes especially if you are doing the same workout over and over again, your body will get used to it and eventually you will plateau because your body knows what to expect. Depending on what you are doing for your workouts, maybe step it up a notch or two on the intensity and see what happens. Also make sure that you are recording everything that you eat...make sure you are getting enough protein and watching the amount of fat, carbs and sodium that you are taking in. If you are going over your sodium, you could retain water.

One more thing, make sure that you are eating enough not only to fuel your body, but to fuel your workouts....especially if you are working out at a high intensity you need to make sure that you eating enough calories.

I'm 5'3 and got up to 148lbs during pregnancy. I got back down to 116lbs in 4 months by exclusively breastfeeding, eating 2000cal/day and walking 15 min/day with baby. Are you breastfeeding and eating enough calories? My net calories are 1500 since I subtract 500 cal for breastfeeding.