I'm about 4-5 weeks pregnant they did a sonogram but they see only the sac no baby.. it's this normal? my hcg levels keep going up rapidly...

Too early : You can not see a baby via ultrasound until about 6-7 weeks. It's too early.

Yes: The gestational sac is the first thing to appear. In a few days, likely they would be able to see a fetal pole, or the beginnings of an embryo. After that, a few days later, a heart beat. Sometimes unfortunately a sac appears but then no fetal pole, so make sure you have a follow up ultrasound to confirm an intrauterine pregnancy. Good luck!