I Haven't Started Using Anything New Or Going Anywhere New

I haven't started using anything new or going anywhere new but I keep getting red bumps on my arms and hands. almost like mosquito bites. some are big some are small... and some come in clusters. they will appear in just a matter of hours no matter where I'm at. they itch like crazy but will go away in about a day and then new ones will appear in a different area. what could this be?