See another doctor that actually gives a sh*t. One that will test you for common things that cause such things. When a doctor answers a question like that they are just blowing you off which is basically them saying "Screw that, I'm not bothering with that, just give me your money."
...Before you go hating on me/someone saying that consider that you are paying a doctor for their service. Even if you have insurance that's good enough that you don't pay anything, they still get paid, and the real fact is that the reason we go to doctors if for them to address our health concerns and not just give us "A" answer, we want THE answer. If they're not addressing your concern and just giving you 'an' answer....you either need to address that with them that you're not satisfied with their answer, which is a pretty lame answer if they didn't even look at it, witness it, and test you for anything, and if they're not going to satisfy you you it's a free world and there are plenty of doctors in it with us. Choose and go to another one that will satisfy your concerns.
This could just be a nervous tick as they call it, but it could also be due to a brain tumor, Parkinson disease, diabetes maybe....it could really be anything. I'd want an answer too. Without tests you'll never get an answer. Your choice is to get your regular doctor (if you have one) to really find out what the cause is, or find another one that does.