Bumps

I have developed these strange bumps on my left foot.  They have stayed only on my heel though.  They only develop when I sleep though.  They also itch.  At first I thought a mosquito or spider bite but insects and arachnids aren't smart enough to bite in the same area three times in a row.  Any ideas on what this might be?

Have you been in an pools, lately?  Any Blacks around the pool?