It sounds like the rash you have is probably urticaria, or hives
. This is a very common skin reaction characterized by raised, red, blotchy spots that come and go over large areas of the skin. Often, hives are quite itchy, but other times they may just be flushed and warm, as in your case.
Many times, hives can be brought on by a specific allergic reaction, such as exposure to a specific environmental substance that your body reacts to. For example, people with certain food allergies may break out in hives when they eat those foods.
However, it is also possible to break out in hives because of non-allergic triggers, such as stress, heat or cold, or exercise. This type of non allergic hives is called "physical urticaria." Generally speaking, we don't totally understand why physical urticaria occurs, but this doesn't mean that there is nothing that can be done about it! First of all, it can be helpful to identify which events or environments specifically trigger your hives and then try to avoid as many of those as possible. For example, if drinking alcohol causes you to break out, then you should avoid alcohol. Second, there are also medications that can be taken to help prevent breakouts.
Start by seeing your primary care doctor
or an allergy doctor, who can give you more advice.