Girls, such a strange situation with the cycle, maybe someone has enco…

Girls, such a strange situation with the cycle, maybe someone has encountered it. Periods have always been stable every 28 days. But the last few months they've become 26 days. Now there's a delay of 4 days, the feeling that they will start soon and my stomach feels tight like when they come. And they don't. I took a test, it turned out negative. What could this be? A malfunction? I'm not breastfeeding. I'll see a doctor in a couple of days.