Is that the only time it's happening? For months I've been dealing with this dizziness but it's not just when I suddenly sit up. It really drives me crazy though and is one of the reasons I've started seeing a doctor for anxiety. I had so many medical tests done before anyone said it could be anxiety and nothing turned up in any tests to explain why I was dizzy. It was really frustrating. It also doesn't help that it's a common side effect with most medications too.