Since she turned four months, every day nap ends at exactly 30 minutes

Our daughter used to nap an hour and a half at a stretch. Over the last two weeks every single day nap has started ending at 30 minutes, almost to the minute. She wakes up crying, not refreshed, and by evening she's overtired and we're both done. I'm on maternity leave, so I'm with her all day and I've tried the usual: darker room, feeding her back to sleep, patting. Sometimes it buys another twenty minutes, mostly not. Is this a phase that sorts itself out, or did you have to change something? Nights are still okay, thankfully.