3 days late

So I had my first pp period at 4 weeks post partum. Typically my period cycle is 26-28 days and is usually always on point. My second pp period I got was on cycle 39.

I’m 3 days late according to apps, on cycle day 29. I took a preg test and it cane back negative.

It possible that my cycle length is changing and extending? Anyone going through this?