I don't think the period pains argument is very relevant here. She hasn't started her periods (or the OP hasn't replied to those mentioning that, that it is an issue)
If a girl does have to take a day off school for period pains, then I think it possibly could be treated differently,as not illness as such.
If ever my children are off (and it is rare I'm glad to say) then we always start from the point that they may well go into school at lunchtime. being off in the morning does not automatically mean all day off - and I have taken child in at lunchtime. In this case, you could still say no party (being at school for afternoon is probably enough for a newly recovered person) or be more flexible as illness no serious and school attended.
Equally if child has been peaky in the morning, I always point out that a day off school will mean no scouts/dancing/swimming party etc in the evening. Usually I suggest child goes to school and 'tries' and I will collect if necessary - right pain as I'm at work, but in most cases the 'trying' results in a full day at school and 'illness' forgotten - not to say I don't believe they feel peaky, just that in the early morning it's easy to feel rough with a cold or whatever, but getting going etc often makes you feel better and quickly forget it - cruel mummy. (Not talking about sickness/diarrhoea etc obviously nor anything g contagious, but headaches, tummy aches, colds)
If too I'll for school, too ill for party. I'd say she needs the time to fully recover and a party might 'set her back'.
Remind of the rule any morning when illness/day off school is on the cards. Be consistent.