Thick lining on the ultrasound, no period...

Hi, I'd like to share my story.
12 weeks ago I gave birth to a precious son via C-section. Still no period since the birth.
2 weeks ago I had a check-up with the gynecologist, on the ultrasound she could see that my uterine lining was thick and that I would get my period the following week (so 1 week ago). Still no period though.. Took a test yesterday, it was negative.. What could it be? I do have period cramps and back pain for a week now.. Can it take this long for your period to come if a thick lining was already visible 2 weeks ago?
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