6 weeks gestational sac but couldn't see baby quite yet

I'm really worried. Went to ER for back pain and they did blood work and ultrasound. The back pain turned out to be just the way my uterus was ... The bloodwork came back my hcg levels doubled up since the last time they checked but the ultrasound there was a gestational sac but not really baby seen yet... The nurse said she said I should be around 6 weeks and said it could just be too early.... Anybody else have anything like this happen before? I'm worried about there being no baby 😣😥I need encouraging words