No you are definitely not being unreasonable - I think you are very astute to actually acknowledge that this goes on. I am not a teacher, but if you have any views on the following I would very much like to hear them.
Without going in to any boring details, myself and another mother who has a very forceful and controlling personality fell out. Coincidently she is very involved in the PTA.
On a couple of occasions I have observed her manipulating a parent/child against my little boy as a way of getting back at me.
It is very hard to deal with because if someone has a problem with you you can deal with it, but it is heartbreaking when it's taken out on an innocent child.
I think in a way because she is meaningless to me, she has to try and get my attention or wants me to react.
The way I deal with it is to just literally carry on as normal. I suppose I am quite brazen in that I do not scurry off the playground, I stay until my son has finished playing with his friends. If any poison has been spread about, I think the proof is in the pudding, and people with any nouse will eventually see what she is doing, and that actually we are okay.
It can be hard though, and I wouldn't like to know how many man hours I have spent thinking on how best to deal with the situation.
My son is only in Yr1 and I had absolutely no idea what a minefield a lovely place like a playground can actually be.