Are aircraft being fitted with brighter anti-collision lights?
Its possible they are just fresh halogen bulbs and brighter than others you have seen but your key line is: 


  against the low overcast clouds


Low clouds, fog, and the like give strobes something to reflect off of so they seem brighter in a localized way. I cant find a great image of it but this video showing the graphics updates in X-Plane 11 show the phenomenon 
fairly well. On a cloudless night the strobes (an most aircraft lights) have little to reflect off of so they seem darker when looking at them. If you watch departures and arrivals in similar conditions you will also see the landing light throw a similarly "visible" beam. This is all somewhat similar to how laser light is visible in fog.