It sounds like you have quite a few of these areas on both of your arms! Because of this, I definitely think it would be a good idea to have them checked out. You could see your primary care doctor
, or go to an urgent care center, for help.
One possibility is that these are areas of burned skin from being in the sun. Even 1 hour of direct sunlight can be enough to cause burns, under the right circumstances. In addition to causing the skin to be warm, reddened, and painful, more serious burns can also cause blistering like you are describing.
Another possibility is that these are hives
, which are a sort of allergic reaction in the skin. Hives are red, blotchy, raised areas of skin that are usually intensely itchy. More severe hives can lead to blistering, although this is more unusual. Hives should respond to benadryl, but they may sometimes not respond very much.
Your doctor can take a look at the areas and determine what exactly is going on, whether it be sunburn, hives, or something else entirely. They can also help you decide whether any medication, either oral or topical, is necessary. Good luck!