You may need some more "set up" done on the neck. If you measure them are the e and b strings actually higher off the fretboard than the other strings? That could require some more "set up" work. If you nut is filed wrong that could be the answer. Go back to the shop that changed your strings and lowered the action and talk to them about your e and b string problems. If those strings are too high off the fretboard that could require filing your nut a bit. Probably best assessed by a skilled luthier who is looking at the instrument and talking with you about what your problem is.

Is your pain, when you finger the b string of the Am chord, from the arthritis or is it from a lack of callouses? Those narrow strings do cut into your finger tips more.

Do you stretch before you start playing the guitar?

Does the pain subside after you warm up your fingers by playing guitar for a while?