One of the reasons it is affecting your self esteem you have already identified: you are repeating it in your head. You need to break that pattern and override it with some positives. To override it, you need to repeat them and to believe them.
Say to yourself, "I am a beautiful person."
Think about times when others have said anything complimentary to you. Repeat them too.

Another reason it is affecting your self esteem is it came from someone you know, and whether you value her opinion or not, you are still bothered by her thinking this about you.

Most women self monitor their appearance a lot. There is often a little voice checking that we look OK. "Is my hair tidy? am I slouching? Does this shirt go with that skirt? Is my make-up tidy?"
This adds to the negative if we are already feeling bad about ourselves, say if you are tired, and haven't done your usual routine for getting dressed up. Quite possible if you have a young baby! It is OK to be relaxed, and to slouch, and to wear comfy clothes. It really is. Especially when looking after a baby. We don't expect someone working in a field hoeing to look pretty. A mother chasing after a little kid is working just as hard too. She may even get sweaty and have baby vomit on her shirt.

So you need to override that voice, and make sure that you are not requiring yourself to look good all the time. Try overriding this with something ridiculous, to put her opinion in the right place (not very important!) Picture yourself in baby messed clothes, holding your baby and standing proud, and strutting down a catwalk.
Ta dah! You are great! you can look confident in any situation! Maybe she is in the audience and envious and you walk by without even noticing her opinion because you are happy.

You can still dress up when you want to, but if you find yourself monitoring what you look like, make sure you counter it with something positive:
"My biceps are getting really strong from holding this child."
"I am really happy to see my child do something new."

And smile about yourself. You are doing really well.