I suggest waiting until the day after your Ultherapy before you do hot yoga or exert yourself in any signficant fashion to avoid increasing the amount of swelling that could develop. By the next day it should not interfere unless you are very swollen.
There is no downtime following Ultherapy. Most patients can resume normal activity, including exercise immediately following the procedure. Some mild facial tenderness or sensitivity is common, but it will not interfere with your ability to do any type of yoga or aerobic activity. We have patients who leave the office and head right to gym.
Most patients are able to assume their normal activities soon after Ulthera treatment since there is no real "downtime" associated with the procedure. We allow our patients to exercise the following day, however, I would recommend asking the physician treating you what his/her personal recommendations are regarding this. Hope this helps!