Aibu? Party invite

[Happysea]

Hi I may well be being totally unreasonable but I'm perplexed at what's going on and feel very sorry for my daughter. She was 7 yesterday. Last weekend she had a party that involved inviting 6 children to an activity and then to a pizza restaurant for tea. I've since found out that one of children had a party the weekend before for their birthday (my daughters invites were sent out over a month ago) with the same number of children but didn't invite my daughter.

My daughter shares her birthday with another of the girls she invited. I've found out via Facebook (I know, I know!) that this friend is having a party tomorrow with quite a few children in my daughters class but my daughter again isn't invited. I understand that these children may not have wanted to invite my daughter (fair enough) but why come to my daughters party last weekend? More so, I would feel awful as a parent sending my child to a small party gathering one week (it took 4 hours in total and included me driving them all there and back) but purposefully not inviting that child back to their own daughters? I do suffer from anxiety so I may well be unreasonable. I just want to know if this seems normal?

I don't like kids being excluded, there's being excluded because only a few are going and then there's the other type, the unpopular kids not being invited just because, that's so wrong.

Not read all the replies but aw no, can see where you're coming from as you want your child to be involved in all social stuff.
However, with a 9 year old and a couple of years older child, been through many parties in the past.
You can't always feasibly invite everyone, however you may want to. Costs put paid to that unless you're loaded. I'm unfortunately not.
Don't take it so personally. Let your child know breezily "aw, sometimes we can only invite a few people". They happily accept it if you don't make a big deal.