9m pp, no period, but not pregnant?

I’m a week and a half late, I’ve always always been regular. After having baby I bottle fed, so my period came back at 6 weeks and its been every 27 days, like usual, since then. Now all of a sudden I’m missing my period, no pms, no cramping like usual, it’s been a week & a half, I’ve tested 2 days ago and it was negative plus we’ve been extra careful. No diet changes, no extreme stress... has this happened to any of you ladies? I’m a FTM so I don’t know if this is usual. I would’ve thought this could happen more so in the first couple of months but not 8 months of regular periods?