9 days late but not pregnant?

I had sex like 3 weeks ago and my period is 9 days late. I took a test last week and it was negative. I’m also on a liquid diet after surgery so i assume that’s why i’m not getting my period. It was weird i was cramping and stuff like i was on but no blood ! probably because ive lost 20 pounds in a month...should i take a test tuesday morning which will exactly be 3 weeks since i had sex just to double check or does it sound like i’m not pregnant?