I have a habit of "biting my cheeks". I find myself doing it without knowing it. Is it "normal" and how do I stop?

Situational habit. I used to do the same thing when i would find myself in intense competitive situations. I gave myself something else to do in order to save my cheeks. Sugarless gum. I no longer chew much gum but the cheek biting habit is gone. Consider current medications, dietary deficiencies, anxiety, malaligned or poolry occluding teeth as causes. A visit to your md, or dentist might be helpful.
Anxiety!!! This could be a sign of anxiety. The stopping it might no be as simple as it sounds, you would have to be constantly aware of the behavior and slowly modify it until you are able to stop it before it starts .
Could be a problem. Although usually innocent and likely due to anxiety, sometimes it can be secondary to medications. Reglan, antipsychotics, antidepressants, anti-nausea medicines, and miscellaneous other meds can cause tardive dyskinesia. This is a potentially disfiguring involuntary tic that may not discontinue with medication cessation. Dental problems?. Compulsions are typically responsive to ssri, cbt, buspar (buspirone).