This sort of twitching (very fine fasiculation) is extremely common, even without any obvious cause, and since you've had work done in the area served by the same nerves that serve the upper lip area, it seems this could be related to that. The nerve may be slightly irritated, and this sort of twitch, as you say not visible to the casual observer, feels a lot more annoying than it looks. It should resolve on its own, but this can go on for weeks at a time. All your exams have been normal, so unless this becomes so annoying that you might consider nerve ablation (destruction of the nerve causing the twitch, usually by injection of alcohol), which could leave you with numbness, inarticulation or even a visible droop in your lip, I'd recommend leaving it alone. While it is undoubtedly bothersome, it will probably resolve on its own. If you continue to have tension headaches in your neck (I know exactly what you mean by that), your doctor might be willing to prescribe a small amount of a muscle relaxant or sedative that would serve the same purpose (muscle relaxant like Skelaxin or sedative like Valium) which possibly might cause the twitch to stop with one or two doses. It could be worth a try if it doesn't go away on its own.
I hope this is helpful. Best of luck to you.
John Kenyon, EMT, CCT
Non-invasive cardiology tech, Emergency and Critical Care technician, Critical Incident Stress Mgmt. specialist