That really stinks! It hurts when people act that way.
It's hard, but what you have to do is remind yourself that this might not even have anything to do with you. I'm not saying you're imagining this at all, but it is possible that they do this with other parents also.
Try to keep in mind that there could be many different things going on in their lives that would cause them to distance themselves from others. There could be health concerns (many she has really bad acne or something). She could have recently gone through something traumatic that is making things more challenging socially. Maybe they're going through a rough time in their marriage. She could be embarrassed about something. There are just so many things that they could be going through that could have nothing to do with you.
It could also have something to do with you. It could be that they are upset about something or that they have heard something or who knows. Unfortunately, if that's the case, you just kind of have to let it be and do your best to not care. It's not easy, but it's what you have to do. You have to come to a place where you are not going to care when other people don't like you. You are just going to choose to not care.
I've been through this a bit because one of my sons is on the Autism Spectrum and has some weird behavior issues. There's one person I'm thinking of at church who is always giving me dirty looks and is definitely judging me. It truly all started one day when my son would not stop coughing (it's a tic, and it is sometimes a very disgusting sounding cough). She gave me a disgusted look and has snubbed me ever sense. I don't go out of my way to talk to her, but I have been polite and said hello and good morning to her. She sometimes says hi and then sticks her nose up in the air. I do my best to just let it go.
My point is that it does get to me from time to time, but I try not to think about it. Some people are never going to like me, and that's just the way it goes.
Just keep being polite and live your life and do your best to not let her (or them) get into your head.