Actually, I can see where you are coming from OP.
Guess I am a self-confessed people pleaser too. And fact that other mothers are still on FB and presumably haven't had a status change, can leave you feeling left out and that is never a nice feeling.
On the other hand, consider-
Is there a rationale alternative explanation for this? Is she having a general clear out, as some have suggested. Could it have just been a mistake? As things stand, you have no way of knowing but are assuming the worst, and taking it personally.
Does it really matter? Are you overinvested in what some random school gate mum thinks?
School playgrounds can be great meeting places, or hellish dens of intrigue - depending on circumstances. But why do something that makes you feel bad? Limit the time you spend there. Quick drop off, quick collect, and spend the time doing something you enjoy, or engaging with (real) people that make you feel more positive.
Life is too short to waste it on FB fuelled petty intrigues. Take time off from it. You can always pick up again if you want to, and when you are in a better place. If you stay around people who engage in this (if she is) and who make you doubt yourself, and feel bad, it will only make you feel worse.