I mean you could be having twins. I do know when I was PG, I found out only at 3 weeks too. I also looked pretty big by 3 months. BUT going into my 4th and 5th month my weight was kind of steady. Then by my 7th month I started gaining alot faster again.
Hcg levels aren't a good indicator. At 5w3d my levels were 11000 at 5w6d they were 32000. I only had one. But if you are thinking twins have your doctor do an ultrasound to see.
Weight gain like you describe is most likely from fluid.
Your numbers are rising at a rate that is reasonably consistent with a singleton (only silghtly on the high side of normal range) but twins can't be ruled out. It's also possible that you ovulated earlier than that you thought you did.
When is your US?