I would not go straight to something serious like leukemia. The article that Anniebrooke linked is a good one and has some very plausible reasons why it may occur. To me, from your description, it sounds most like Vasovagal syncope which while annoying is not serious. As suggested, she should talk to her doctor. The issues of low vit. k and potassium and high uric acid are things to sort out. Low/high is not the greatest description as it could be slightly outside of normal and be termed high or low when it isn't really impacting anything. So, you need her doctor to give you the numbers and the range of normal to see where she really falls. Then through nutrition and supplementation, that should be able to be fixed. Let us know when she goes to the doctor.
As the following article says, there are a lot of reasons for nausea after sex.
https://www.healthline.com/health/healthy-sex/nausea-after-sex#anxiousness
I don't think the blood tests answer the question. I'd guess it's one of the other things on the list in the article I referenced. Maybe vasovagal syncope, or endometriosis? She should take the list to her doctor and get some of the things on it ruled out.
You say "Is this a red flag that I need to address?" but this is your fiance's medical condition, not yours. It's sweet that you are concerned for her, but she is the one to talk with the doctor, not you.