Hello Megan

It would be impossible for anyone here to even guess at what is causing your itchy bumps.  Rashes are sometimes like solving a mystery.  You need to examine all the information. 

Did the spots suddenly appear or did they come on gradually?  How many do you have?  Is the rash spreading?  Are you allergic to anything?  Have you been outside and have you had exposure to any sort of plants, weeds, insects, animals?  Are you having any other symptoms?

Here is an article on what to do if you have an unexplained rash.

Are you itching so much that you are bruising the skin and this may explain the purple color?  Any chance you were bitten by a spider? 

Here is some information about spider bites.

Please know that I am not a doctor.  But I would be a little concerned about the bites or bumps changing color to purplish.  I really think you need to see your doctor on this.

I hope you feel better soon.  Let us know what you find out from your doctor.