I am having trouble losing more weight?

Okay, I just had my second child 2 months ago and I am now back to my pre-pregnancy weight but I'd like to lose 13 more lbs. For the past 3 weeks I haven't lost anymore weight. I am 5'2", before my first I was 107 and got up to 150 lbs and at 12 weeks post-partum I was back down to 108 with diet alone. In the interim years I got a nice cushy job with plenty of eating out so when I got preggo this time I started at 122 lbs (had been that a good year) and reached 154, now I am back to 122 at 8 weeks postpartum. Am I having more difficulty getting back down under 110 because 122 was the result of legitimate weight gain (not pregnancy weight gain)--yes my belly size is staying the same? I am cutting calories and exercizing and thinking of doing aerobics to try and cut the rest of the weight -- do aerobics really work? What dvd (trainer) works at cutting down fat?? How much should I start with if I have had no cardio training?? I did a you tube aerobics workout for twenty minutes and 48 hours I can still hardly walk so I think I might have overdone it. Do you tend to see results relatively quickly (within weeks) ??? I am totally new to this aerobics thing so I apologize for the fatuity.