why am i not losing weight?

It's hard to answer these kinds of posts without more details. It's only been a couple of weeks, so it could be a natural stall. Especially since you're post-partum and your hormones may still be a little wacky.

http://community.myfitnesspal.com/en/discussion/10084670/it-is-unlikely-that-you-will-lose-weight-consistently-i-e-weight-loss-is-not-linear/p1
http://community.myfitnesspal.com/en/discussion/10196160/scale-stress-syndrome/p1

Other than that, the most common problems we see come from underestimating calories eaten and overestimating calories burned.

Opening your diary might help to get you more specific advice if you're comfortable doing so.

You're logging everything you eat? Including condiments, cooking oils, veggies, cheat days, etc? Are you using a food scale, measuring cups, or eyeballing your portion sizes? Most people can be off in their estimates by several hundred calories when they eyeball portions. Measuring cups are better, but a food scale is going to be the most accurate.

http://community.myfitnesspal.com/en/discussion/1296011/calorie-counting-101/p1
http://www.myfitnesspal.com/topics/show/1234699-logging-accurately-step-by-step-guide/p1
http://www.myfitnesspal.com/topics/show/1290491-how-and-why-to-use-a-digital-food-scale/p1

And make sure that you've calculated your calorie goals appropriately. Remember that these are just estimates. You may need to play around a little to find what works best for you.

http://www.myfitnesspal.com/topics/show/1080242-a-guide-to-get-you-started-on-your-path-to-sexypants/p1
http://www.myfitnesspal.com/topics/show/819055-setting-your-calorie-and-macro-targets/p1

If you're exercising and eating back your earned exercise calories, be sure that you're using accurate estimates of your burn. MFP and gym machines have a tendency to overestimate certain activities, which can cause you to eat back more calories than you need to. Even a heart rate monitor isn't 100% accurate. If you're eating those extra earned calories it might be a good idea to eat only 50-75% of those.

And there's something to be said for the fact that some people just burn fewer calories than the generic equations predict. If you're an outlier due to size, age, or medical issues, then it may be best to check with your doctor or get a referral to a registered dietitian who can give you more specific advice.

Congrats on the little one! There are a lot of reasons why this type of diet/exercise app works, and why it may not be working. Here's what I'm learned during the last few years that I've been on MFP. (Keep in mind that everyone is different and what works for me may not work for you, it never hurts to consult your doctor and/or professional trainers and nutritionists.)

1. Be honest with the app. Log EVERYTHING you eat and drink.
2. Measure/weigh your food. You may think you're eating an appropriate portion of something, but if you don't have a kitchen scale and good measuring cups, you could be way off base.
3. Don't weigh yourself more than once a week. We all have weight fluctuations throughout the day, week and month, especially women. If you weigh everyday, it can be extremely discouraging because you're not seeing the larger picture.
4. Hydrate, hydrate, hydrate. No more soda, juice or energy drinks, load up on the water. This can make all the difference.
5. Don't try to deprive yourself of any major food group/type, that will only make you crave it more. You should be able to eat a little bit of everything you love in moderation. (Again, see #2)
6. Let yourself have a break once in a while. I usually give myself one "cheat" day per week, and I find that after a few months of logging my food, I don't want to go crazy on those days, I just need one day each week to not worry about food labels, I want to enjoy myself. I plan my cheat days based on my social activities, it helps me stay on track if I plan ahead.
7. Forgive yourself. Everyone has bad days, or even just a bad meal. There will be days where you start with the best of intentions, but fall short. It's ok, everyone on MFP is still hear to cheer you on and get you back on track.
8. MFP for me is not about dieting, it's about changing how I think about food, I want this to be a permanent change in habits and mentality, and I hope you can see it that way too!

I really hope this helps, I just had my second baby a month ago, so I'm starting up again after a long break. How long has it been since you had your baby? Are you nursing? Don't forget, our bodies undergo huge physical and hormonal changes during and after pregnancy, and it can take a while for everything to get back to normal.

Good luck, you can do this!