Odd, sounds like time to do a full-out testing for everything; ammonia, nitrite, nitrates, alk, PH. If your corals are okay, probably isn't calcium. For safe measure, check your salinity (make sure your test instrument is working correctly), and obviously temp.
---Even if nothing looks like a problem, I'd do an immediate water change--25% or so. And keep doing them for the next few days.
Water changes are the panacea of all problems.
I hope it's just your PH. The good thing is the effect from the buffer is immediate.