Based on the design of the square taper bottom bracket I think there's basically no chance that the spindle moved, especially if there's no play, so I think the only reasonable answer is that the crank has "settled" onto the square taper as you've ridden the bike, moving the chainring closer to the chainstay.

What spindle length does the UN-72 you installed have? From this page on your crankset:

VeloBase.com - Component: Shimano FC-B124, Biopace Touring Triple

It would appear that you'll need one that has a pretty long spindle length, or with an asymettric spindle as the drive side apparrently should have a 5mm offset. If your bottom bracket doesn't comply with that, it's not surprising that your chainring is hitting the frame.