Hi, My name isXXXXX you for using Just Answer. I am happy to help you with your concern.
Your thumb contains voluntary muscles, so from time to time it is going to twitch and probably there is nothing to worry about. Stress can defiantly cause twitching.
You may have a magnesium deficiency. You can have a simple blood test done to check your level. You can increase your intake of magnesium rich foods such as almonds and green vegetables, especially spinach.
You may also have a repetitive motion injury. If you are on the computer a lot, play video games on a regular basis or any other repetitive motion then you could be damaging the nerve. Resting from the repetitive motion is your best bet along with taking an anti-inflammatory medication like ibuprofen or aleve. (You can either take 400mg ibuprofen three times a day or 1 Aleve twice daily with food) You may also want to get a brace (called a thumb spica splint) that can be purchased at medical supply stores.
There are a couple of serious diagnosis that it could be, multiple sclerosis or ALS. So if this does not get better in another week then it is time to see your doctor and have it examined.
If you have any further questions, please let me know. I hope you get relief soon. Have a nice day.