The luteal phase can shorten. It's governed by progesterone, which is produced by the corpus luteum, which in turn was developed by a surge of estrogen. So, if, for example, the surge of estrogen was weak, though you may have ovulated, the corpus luteum wouldn't be strong enough to produce enough progesterone to sustain the lining. This can happen as egg quality diminishes. Or it can happen sometimes, just because. Just as sometimes you may have an anovulatory cycle.

I wouldn't worry about it unless it becomes a regular occurrence.