For the last month, any time i use my right hand thumb to do anything, like twisting open a bottle or opening a bag, it starts twitching and spasms for a few seconds. there's no pain, but when it does this i can't control it. what might be the cause?

This type of temporary thumb spasm can be due to stress or fatigue. When it quickly resolves, it is unlikely due to a serious problem. If the spasms worsen, seek an evaluation by your physician. You likely will need laboratory tests to rule out a metabolic problem and a neurological examination. Magnesium deficiency can cause muscle cramps and it's a very safe and inexpensive to try.