Try drinking more water to keep your bp up. Try turning the water off while you wash your hair.
The hot water causes your blood vessels to dilate, lowering your blood pressure, which can lead to a person feeling dizzy or faint. When a person feels faint, they can also feel nauseated with it from the blood pressure issue.
I recommend if you want to continue with the hot showers, try it with a shower chair and then continue sitting for awhile afterward until your body will be better able to handle standing up. I know from first hand experience that throwing up in the shower can be a real mess to clean up.
The alternatives would be to try hot baths and see if you do better with that or start taking luke warm or tepid showers or baths instead.
I learned myself when I have a migraine, it's best not to take a hot shower during one, so if you get those, avoid showering during the headache.
Well, what makes me feel faint and dizzy, etc. in the shower are low blood sugar + hot shower. A low blood sugar can make you dizzy, nauseous, and like you're going to faint. Eating something helps. This is usually only an issue if I've been sick and not eating as much for several days. Really hot showers contribute to this by causing your blood to go to the skin to help cool you off. If enough blood gets redirected to your skin, you stop having enough for your brain and start feeling dizzy.
I would suggest trying to eat a little something before showering, and try showering with the water a little cooler. If that doesn't do it, you should see a doctor to make sure nothing worse is going on.