What Causes Twitching And Numbness In Arms And Loss Of Balance?

For a while now I ve been experiencing dizziness and numbness in my left arm after I stand up. This doesn t happen every time, Ii was concerned so went to the doctor and they said Ii had low blood pressure which explained why I felt dizzy but not the issue with my arm. I asked about that and the doctor looked puzzled. He said not to worry about it as I was young (I m 28). This has now changed and the numbness is now a twitch and I have no control over my left arm after walking a couple of steps. I lose my balance and I ve slipped over a couple of times. I m worried that it s something serious as this has also happened while sitting. I feel as though this is getting worse rather than better and I m not sure what to do.