With my older one, I found that when she was getting to be more mobile (learning to roll over, crawl, etc) and was expending more energy then she needed more naps, but after awhile she got used to it and was able to stay up longer again. I think their bodies need to adjust to the increased activity with more sleep and then once they are used to it, they no longer need the extra sleep. As long as you are following cues, I'm sure everything is fine.
