At 18 months, my DD went from 1.5hr naps (her nights were 11.5 hrs) to 1 hr, because like yours, her sleep started being disturbed (in my case, it was bedtime rather than naptime, but I think, overall, it could be same thing, ie needing a little less sleep in 24 hours period). What I did was push nap 15 minutes later, and wake her at same time, so routine went from 12.45 - 2.15 nap to 1.00 - 2.15 nap, and I kept the same bedtime.