healthy dog with random tremors

My very healthy, very active, lean, 75lb, 3 year old staff terrier mix had some random head tremor thing going on this morning. It lasted about 10 minutes...her head was shaking kind of like katherine hepburn's used to (also kind of made me think of parkinson's or those bobble head dogs you mount on your dashboard). She's been acting completely normal and hasn't done or eaten anything out of the ordinary. I noticed it after I took her out and fed her. She was lying on the couch with her head down when I noticed it, and when I picked up her head with my hands and held on to it, she continued to tremor. This actually happened one other time about 3 months ago....much the same, only lasting around 10 minutes. She does't seem to react to the tremor, but she does seem a bit confused by my immediate concerned attention when it happens.

Oh, and she isn't shaking it herself, this is uncontrolled but steady movement. Any thoughts on this would be appreciated. I don't think it's anything serious, but I can't be taking any chances with my girl!