Not baby related. 3days late

So I had my baby March 12. I had my period 4 weeks pp. Typically my cycle days are 26-28 days, always on point. On my second pp period it took a little longer, on day 39.

I’m currently waiting for my period I’m on cycle day 29, 3 days late according to the apps. I’ve took a preg test yesterday and iT was negative.

Could it be that my cycle changed and extended longer?

Anyone else going/ gone through this?