What could cause bumps and spots that come and go?

[nthasama]

I have a two year old son.  Yesterday on his leg I noticed what looked like more than a dozen bug bites.  They were raised red bumps.  3-4 hours later I was at my parent's house and wanted to show them to my parents, but they had changed.  They were no longer raised, just flat red spots.  At that time my son also had red bumps behind his ear that had not been there before.  Now, less than 24 hours later the bumps/spots from his leg and ear are both gone, but now he has flat red spots on his tummy.  What could cause this?  Do I need to be concerned?

[purple1234567]

It sounds a little like hives to me- I'm posting a link to another post with similar question that one of the dermatologists had answered, the information she gives should help!
https://www.medhelp.org/posts/Dermatology/itchy-bumps-that-reappear-then-disappear/show/1383133