Your DD is starting the process of dropping from 2 naps to 1. My DD also started at about 13 months and was finally done with her second nap by 15 months - it took a while. Although the sleep books say that most babies drop their am nap, it is not unusual for babies to keep their am nap and to drop their pm nap. This is what DD did and also what lots of my IRL friends kids did. My DD was the same as yours - always took her am nap then would refuse the pm nap. The transition stage is difficult - but what I did was very slowly move the am nap backwards, by 15 minutes every 3 days or so. In the early stages, I still put DD down for her pm nap but eventually, as the am nap got later, I stopped putting her down for the pm nap. I also moved bedtime earlier both to make up for the lost nap and so she wasn't awake for so long in the afternoon. DD used to nap at 9 am and 1 pm, now she naps at 11:30 am. I could never get her nap later than 11:30 am and this is still her nap time at 2.25! She goes to bed at 6 or 6:30 and wakes up early, anywhere from 5:30 to 6.
Goodluck, the 2 naps to 1 is a rough transition!