No gestation sac at 5 weeks

I was exactly 5 weeks last Sunday and they did an ultrasound because I was having cramps. When they did the transvaginal ultrasound the ER said they couldn't see the gestational sac in my uterus nor in my tubes but they said my HCG level was at 6,000. The ER doctor told me that he couldn't rule out a tubal. When I called my doctor, they had me do another HCG level test and my levels are doubling normally.. Anyone had this haplen? Was it just to early to see the pregnancy?