I swear to God, I have the same thing, except for me the spasms are all down my back and my right arm is going numb near the elbow and my right hand is itchy. It's been like this for a couple of weeks now. My wife has been a programmer for the past 20 years or so and she has this awful thing happen every once in awhile. On those occasions she needs to go for physical therapy, deep electrical stimulation, etc.
When I go out and work in the nursery it goes away, but as soon as I get back on here it comes back.
I guess I need a different chair.......... or less tension ......... or fewer threads.
;-)
Change the height level of your monitor. It works. I went through a similar thing years ago and it mystified the doctor. Someone else told me to change my monitor and the problem went away almost immediately.