Slow/Normal FHR?

help me understand. i called my OB and spoke to the nurse and i’m still not quite understanding. this is my first pregnancy i’ve ever had. i went for my first ultrasound on the 25th and was measuring 7 weeks 1 day, not 8 weeks 3 days which too me is confusing, did i possibly ovulate late? what i really don’t understand and has me almost in tears is the “Slow FHR” we weren’t able to listen to it but we’re able to see their heart beating on the screen, the probe wasn’t sensitive enough he said to listen to it this early. but then it says “Normal FHR”. has anyone else experience this before? im so confused and it’s making me worry.