What causes late rising hcg levels ? i mean i was 6 weeks pregnant but hcg was 148 then it still going doubling normally every 48 hours.but only sac

Early: An HCG of 148 is very early. As long as it is rising like you describe, it's a good sign. You don't expect to see a sac until the HCG reaches 1500 to 2000, and you won't see an embryo for at least another 1-2 weeks, and then just a small blip on the screen. If you are 6 weeks, though, you would expect to see something. Perhaps you are not as far along as you think.