6 Week Ultrasound and Nothing But a Sac

Kathleen
I've been experiencing some mild/achy but consistent cramping since yesterday and, last night after DH and I had sexy time I saw three light pink spots on the TP after wiping. 
I reported to my Dr this am and was brought in for a ultrasound an hour later. Ultrasound shows gestational sac measuring at 6 weeks and yolk sac but no baby. 
Dr is having me come back in a week to do another ultrasound. 
Anyone else have experience with this? Understandably I've been quite upset today.