When I had visited https://math.stackexchange.com/reputation for the first time and seen that it shows lower rep than the main site I triggered rep recalc and forgot about it. When after a while I've seen the effect again I was mildly surprised but triggered recalc again. Now I again see that rep recalc will reduce my rep — and since it would happen for the 3rd time I become wary. Could someone please check that there are no little green men eating my rep? (Speaking more seriously, I don't really suspect a bug, just wonder what's going on.)
There have been a handful of troublesome users posting spam whose accounts I have been destroying. One of those users may have posted a question you answered.