Pregnant or not?

Hello mummies! Eversince I gave birth on June 2021, a few weeks later my menses came. Up till recently my menses have been quite regular. Only that sometimes late/early by 1-2 days. Only lately my menses haven't come for 2 months alr, my back is aching like crazy + I have mild cramps. I check on pregnancy test kit using digital clear blue, I'm not pregnant. Has anyone ever encounter this?