I can't say with 100% certainty that this is what your dd is doing without knowing more details, but if everything is exactly the same day to day, then she is probably making the transition from 3 naps down to 2. I know my dd started doing this around the same time and the days she would take 2 good naps she would refuse to catnap, and the day she had 1 or 2 45 minute cat naps she would have to take a 3rd catnap in the late afternoon. She is now 7 months and it was like that for about 2-3 weeks. Now she is pretty stable with taking 2 naps a day.
I would say if things don't stabilize in the next couple of weeks, you may want to think about tweaking the routine. We just now extended dd's awake time to 2.5 in the morning and 3 in the afternoon now that she is 7months old.
HTH!