Involuntary Brow Contraction? How can I stop it?

The muscles that control "frown line" contraction are actually more than the ones that it sounds like were treated with 11 units. To treat this area it typically can take between 20-40 units (depending on the patients muscle tone, needs, and discretion of the injecting physician). This is usually spread over 3-6 injection sites on the forehead. It sounds like the frontalis muscle may not have been treated or was not injected with enough units of product. This muscle can appear to  push inward to create your furrow (against the frozen treated muscles). The movement can occur when you are not trying to frown, a facial expression that may involve the frontalis. Evaluation by your injecting MD is recommended to see if additional treatment is needed.

11 units is a veeerrrrry small amount, first of all. Usually when people state they have "muscle spasms" after Botox it's because untreated muscles are working harder to counter the effects of the muscles that aren't working as hard, due to the Botox. This would be what's happening in your case. The muscles that can move and work are doing so. There is no nerve or muscle damage. It's simply muscles that can move, are moving. Your options would be to treat these muscles in addition next time, or to not get Botox. Because clearly you have muscles that tend to work very hard (ultimately, these types of muscles will cause a more permanent etching of lines into the forehead and glabella if they are allowed to move), you need more Botox and in additional areas to stop this.