This sounds like a dehydration and drinking water too quickly after wards.
It is said that too much consumption of water can lead to fluid overload in the body and imbalance in the body. Excess water can lead to lower sodium levels in the body, which may further lead to nausea, vomiting, cramps, fatigue, et al. This condition is known as hyponatremia.
https://www.ndtv.com/food/are-you-drinking-too-much-water-here-s-what-it-does-to-your-body-1838118
Dehydration is a possible cause of severe abdominal pain. There is a need to educate the general public about the benefits of adequate fluid intake.
https://pubmed.ncbi.nlm.nih.gov/14764254/
So she needs to drink before the games and
needs to be rotated out for water breaks during the game. She is 8 years. You can lookup her weight and see how much water she needs. It not something that is done in one go and to much water can cause other problems. With no games she should be drinking 8 cups of water during the day(8oz).
Schedule an appointment with the pediatrician before going back to practice or games. Most pediatricians you can call after hours. She is most likely still dehydrated and missing electrolytes, etc.
Now let’s talk about what is appropriate for an 8 year old. Is this organized soccer? If so she should be rotating positions and being subbed out. Specially on a hot summer day on turf. Two games with no substitute and little water consumption is asking for problems. Kid have died because of this. Is she playing one position(defender?). I would have a few hard questions for the coach.