I was told that my pregnancy levels are raising and there's a sac, but so far, no embryo. what does this mean?

This is good, normal: You are describing normal progression of a very early pregnancy. The sac refers to a little bubble of amniotic fluid growing around the developing "baby" which is referred to as a blastocyst before about the 4 week mark then an embryo after that time. The embryo won't be readily visible on ultrasound (just the sac) until about week 5. Pregnancy levels continuing to rise occurs normally as well.