I once worked in a place where a lot of us suddenly had small itchy bites around our waistlines. Very strange. It was in an old somewhat rickety building at a university.. We all talked about it and figured out it must be from rats in the attic area, and it turned out that was true; the bites turned out to be from rat mites, very tiny bugs that we hadn't seen. The building was fumigated and we all got rid of our bites. So, something like this may be going on in your place. Or, possibly fleas, though I seem to remember that they favor biting ankles..