As with any change in your health, the best thing to do when you notice something new and disturbing is to see your primary care physician
to discuss further. In this case, your physician will likely want to ask you some questions about whether you have ever noticed red spots like this before; how long you have been noticing these symptoms with a shower; whether there is any pain or itching associated with the spots. It will also be important to discuss your past health history and current medications, as these two things can often both help suggest what might be causing a new symptoms. Of course your physician will also want to perform a physical exam to see if he or she notices anything specific about your knees or skin to help suggest what might be causing your symptoms.
Overall, it sounds as though the hot water might be causing a localized irritation of your skin. This does not sound serious, especially if the only place that you are noticing this is near your knees. However, before determining that this is not a big deal, you should go over your symptoms with a physician so that he or she can put this new symptom in the context of your overall health to make sure you don't need any further evaluation or treatment. Book an appointment today!