Sometimes I get nauseua, headaches, and impaired motor function when I'm in a warm shower too long. I have had issues with higher temperatures for some years now, I have normal blood pressure and have no outlier health problems, but I still just get those issues sometimes and need to stick my head in a freezer or curl up with my head between my knees and just breathe for a few minutes.
I suggest trying out breathing exercises, drinking a glass of cold water before showers, possibly turning the shower spray cold for a moment or two while you're showering, or maybe even try sitting in the shower for a few minutes just to be quiet and relax and maybe rest a sec? Having a glass of water waiting out of the shower could also be helpful, and you may want to take ibuprofen or some other aspirin for headaches or joint pain to see if that helps c:
If it persists and you are worried, consult your doctor next time you're in or make an appointment for it. Keep a calender and mark each time it happens, maybe keep a journal so you can write down exactly how you feel plus the date and time and circumstances each time it happens?