Am I all of a sudden allergic to bare essentials after 4 years of use??

I have been using bare essentials for about 4 years and love it. All of a sudden I had a mive attack of hives this morning as I put the makeup on. I literally watched the hives pop up instantaneously in the mirror. My brush had some build up on it, that I think could just be old makeup (I've never cleaned the brush), but I'm panicked that it is some sort of freaky spores or something. But even so, why would I break out so bad all of a sudden when I have been using the product for so long? Can anyone relate or have any suggestions? Not to mention... how do I clean these brushes?