Worried! 1st ultrasound at 6 weeks (I’m 36 yrs old)

Karen

I ovulated 7/26 but the first day of my last cycle was 7/6. Today we had our first ultrasound and I’m measuring around 6 weeks. They were not able to detect a heartbeat so they want me to go back neat week for another ultrasound. Going by my ovulation date u guess I’m 5w 5 d. By last period I’m 6w 4d. Should I be worried? They said not to worry yet but I can’t help it. HELP! Any thoughts?😬 this is my 1st time being pregnant. I’m 36.