What could be causing my 2 year old's head tilt?

I am starting to get worried. Took him to the dr. yesterday. She said it could be just strained from an unknown injury (i had taken him to the park a couple of days ago)... but that if it doesnt go away by monday, she will refer him to an opthamologist. She said it could be his vision (or double vision) or something like that. I was baffled. I then read about "torticollis" on the net and got scared of the possibilities. What do you think could be causing this? I am kind of alarmed. BTW, the tilting is not constant, but intermittent.