1st pregnancy ultra sound

Base on LMP(7 Feb 2023) I suppose to be 7 weeks pregnant today. This morning, I notice light brown discharge and past few days had abdomen cramp... Today went for my 1st gynaecology appointment... Doctor did ultra sound and mention that base on scan, I'm only 4 weeks pregnant... Sac can be seen in the scan... doctor gave me dydrogesterone to support my pregnancy and help with the discharge... have to go back in 2 weeks time... feeling worried has I had previous miscarriage in Oct'22. Have anyone went through this b4?