Why is my period late when I have negative HPT?!

Hi everyone,

A little history, I had my second son in September last year, unfortunately due to him having a cleft lip and palate I was unable to breast feed him like I did my first.

8 weeks pp I got my first af and then went on to have 3 normal 29 day cycles.

Fast forward to this month and I decide to start tracking my cycle again and it said my LH surge was on 5th February - now I know it can be up to 48 hours to ovulate so I averaged ovulation to happen around the 6th.

Now I am on cd39 18DPO! But yesterday when I took a hpt at 4 days late it was negative!

Any ideas about what could be going on with my body - it seems strange that I had 3 normal cycles and had an LH surge but period is late and I’m not pregnant?

Anybody had anything similar? What was the end result?

Thanks!