Im 3w+6d pregnant and yesterday in the er i had an hcg level of 121 but, u/s showed moderate fluid in the cul-de-sac. when should i have another u/s ?

In 6-8 days, if well: Your blood betaHCG levels normally would be expected to double about every 40 to 50 hours at this point in your pregnancy. If all is going well, it should therefore take 6 to 8 days to reach 960 to 1800 mIU/ml. At and above these levels, most transvaginal ultrasound exams can detect a small intrauterine gestational sac which is the first sonographic finding in an early normal pregnancy.

See your Ob ASAP...: It would be helpful to know why you were in the ER. In general, you need to see an OB ASAP. I would suggest that you get a repeat HCG blood level in 48 hours to see if it is rising the way it should. I would make a decision about another ultrasound based on the repeat HCG level and your symptoms, and why you were in the ER. Best wishes!