HCG levels at 4wks

Sarah • TTC #1 for 2 yrs, did 4 rounds of femara, 1st IUI on 11/15 & it worked!!! ? Due 8/8/15 with twins!! ?Praise God! ?
Hey ladies! DH & I found out we were expecting our first after 2 years of trying! Had my bloodwork done on Friday at 3wks 6 days, & again today at 4wks 1 day. Dr. said she needs me to come back again Wednesday to get by drawn again because my HCG levels aren't where they want them to be. Friday they were 186 & today they're 640. Every chart I've seen portrays 640 at 4 wks to be good!! What do you guys think? Getting worried!!