I had Ultherapy a week ago. Since then I have been severe migraine, which is something I already suffered with. Any suggestion?

The two are unrelated and I can vouch for that as I have never seen a patient complain about developing migraine headaches. I have seen patients get frontal scalp tingling when they had eyebrow lift with Utherapy but that is a different type of feeling than migraine headaches and it goes away after 2 weeks. 

You will probably want to talk to your neurologist to see if you can get something to help the pain for now, but you could try temporarily increasing your caffeine (the Excedrin has this also) so watch to make sure you aren't too jittery with it.  Sometimes alternating Tylenol and ibuprofen can help for a few days so you don't get too much of either one.  I would consider Botox if nothing else helps.  Even if you do nothing, I would expect the process of inflammation or stress that was possibly triggered by Ultherapy will gradually wear off.  Most nerve inflammation is gone by about 3 weeks.