It does sound like the 4 month regression, DS's 4 month regression began at 3.5 months, I wasn't expecting it at all.
As the first nap is still good I would leave the first A time for now and increase the second moving towards 1hr 45.
I know for many accepting a couple of months of short naps until the tricky phase has passed is the only option but I've also seen some people move quickly on those times and manage to get the decent naps back.
I wouldn't shorten the A too much after a short nap too, honestly I think that was the problem I had, mine liked his A side on the longer side of times and I ended up reducing it way too much. What I'm saying is, if I had my time again I'd increase not decrease.