I believe in God, too, but that doesn't mean that awful things don't happen to us (not His fault by the way!). Vanishing Twin Syndrome is a very likely explanation. It is actually not as uncommon as we think! The only reason doctors know of it is when it is caught early on in pregnancy and then later sonograms show only one baby.
They say many left-handed people are the result of being a twin. Interesting fact to throw in there...
Anyway, it is possible that if there is a 2nd baby, it could be hiding behind the other. Try getting another ultrasound done when baby is a little bigger to see if there is another fetus.
Hmm...cant they check your HCG levels to see if they are extremely high for as far along as you are?
I went in at five and halp weeks and they saw two sacs. One had a yolk and the other had nothing. They said it was probably a twin that didn't make it and absorbed into my body.