Hi Hon, I have a 8wo at home. Please don't stress. I guess it may means nothing. We are having this kind of naps during wonder weeks. Usually I persist with helping him go back to sleep so it's not a nap but just cry out in between a nap. Have you tried lengthening this nap?
We were having not 30min naps by the way but 17', 27' or 37' naps;). I haven't catch any pattern yet except from the fact that if I cannot extend in any way it's UT nap.