Why didn’t 5 weeks show up on an ultrasound?

Hi everyone so I’m currently 7 weeks pregnant, my last period started 6th jan, and I ovulated 27th but I conceived either 23rd/25th/26th. On the 4th of feb I was in hospital with mega pain and turned out to be cysts on my ovaries, I had an ultrasound that found this but I didn’t know I was pregnant at this point. And pregnancy tests were showing up negative by the hospital. On the 13th feb I found out I was pregnant. Now Im currently 7 weeks judging by the date of my last period. I’m just concerned why pregnancy tests were negative only a week before they were showing positive for me, and should I be worried that if I was apparantely 5 weeks when I had the ultrasound that they didn’t see anything?