My DD had a few of these. The GP had no idea what it was but said that as long as they are mobile, he wasn't concerned. Then I think someone on here posted about it and someone said that there's a bit about it in the Robin Barker 'Babylove' book that said that this is really common in babies an toddlers and completely harmless. Always worth double checking these things with a GP of course.
I think DD had 3 or so. Only one remains and it's barely there now.