First ultrasound

So the end of March I took two pregnancy tests two different days at two different time and they both came up positive! Yay! Well I called the doctor and bc I never know the exact time my periods have come bc I’ve always had issues with them I couldn’t give an exact date of my last one so they had me wait two weeks and come in for my first ultrasound thinking I’m atleast 6-8 weeks. Well I went in and they couldn’t see anything but the sac. They said my period could just be off dense I rly have no idea when I had my last one I just know it was somewhere towards the end of February but then she goes or maybe u were pregnant and lost it and just happened to get pregnant again right away. Now I’m all nervous and scared that it happened and I didn’t know. I think something like this happened with my first child when I found out but I’ve never had hem scare me like that. Now they want me to go for blood work and come back in two weeks. Has this happened to anyone else? Should I be so worried?