V.S.
My daughter started taking one nap a day at that same age. At first she did the same thing only napped for 1.5 hours. It was exhausting for me to not get a break, but it took less than a week for her work out her new sleep pattern (nap at 11:30-2:30 and bed from 7:30 to 7:30). What works for me was to put her down for nap/bed as soon as she yawns or rubs her eyes. I also notice that if I put her down for a nap any later than 11:30, her nap gets shorter. The same goes for bedtime. If she goes to bed after 7:30 then she wakes up at least once. It may be that she is wanting to go to bed earlier because she is now napping once, you could try putting her down half an hour earlier than usual and see how she does. This is what worked for us, hope it helps.