Tyler did the same thing off and on from 10-12 mo. We just stayed consistent and went with the flow as best we could. If she's ready to stick with 1 nap, it will probably lengthen on its own or her body will get used to the 1.5 hr nap. If it's just a phase and she isn't ready for it, she'll probably start doing stuff that make it impossible to stay on 1 nap. For us, Tyler suddenly started waking super early AND the nap shortened to 45 min because he was soooo overtired. At that point, we had no choice but to go back to 2 naps. We went back and forth quite a bit between 10-12 mo, and he finally transitioned to 1 nap for good at 13 mo. HTH!