Suggest Treatment For Sudden Black Outs And Stiffness In Muscles

I've been having blackouts a lot lately when I stand up,recently I had one and my hand started twitching then the muscles felt stiff when it stopped. This worries me because my balance has been off as well. I just had my thyroid removed and already did the radiation treatment,in the past 2 weeks I started partial synthroid meds. I don't know if I should be concernedp