Michaela I have the same issue.
It only appears when I wear them for and extended period, like you, about 2-3 days continuous.
Seems it's similar to say hiking for a few days and your boots get wet. Since they don't dry out your skin softens from the constant moisture and you either get a rash or small blisters. The sweat the builds up between the forms and your skin does the same thing. When that happens I have to stop wearing them for a few days and make sure everything clears up. Even if I wash the forms and shower if I wear them again before the bumps disappear it recurs quickly, much more quickly than the first time.
Now I don't wear them for that long without stopping and washing them and my skin and letting everything rest for a few hours, usually overnight. Since I began this routine I haven't had it happen.
