Longer third nap??

So I read a post and curious to try help her out. Her baby’s naps go 1.5 hours, 1hr and the last catnap has turned into a longer nap. Wondering why? And what to do about it? First two naps were already that long. I suggested maybe not feeling well, teething, more stimulation before that last nap? She didn’t respond to those parts in particular so I’m not sure if she’s ruled it out or not.

Anyone have any clue to why a baby would start taking a longer third nap. Baby is 6 months