Dermatology, (the section of medicine that deals with skin problems) is so complicated that it would be impossible to give a particular reason for a rash over the internet. There are so many types of "splotchy red rashes" that there is no way for anyone to determine the cause without someone actually *looking at* the rash.
Having said that, there are a few *basic* sources of rashes that will be listed here, but you *must* see a doctor for a true reason!
Possible causes: Exposure to something you're allergic to like: A medication you're taking A plant or animal you're allergic to Eating something you're allergic to
Also, there are a few disease processes that cause rashes such as: Measles of any type "Fifth" disease and so on.