Solar Activity During May 13th
On the face of it, the Sun's output does look fairly regular; there is no sign of any storms, and the Solar winds do give the impression of being fair to moderate. The probablities also do not show anything of significant value in the physical sense at least.
However, that isn't to say that things were quiet, by all accounts it was quite the opposite, although it wasn't obvious to the naked eye or the awareness. Looking back on some of the reports from Spaceweather's Archive, there was activity but low level.
An alert for low level storm activity during the 24 hour period after May 12th's Storm front, had been issued, so I suspect that much of what was happening may have come from that.