Not a physician speaking here, but someone who works in a health-related field: Couple questions first...how much time had you been practicing with the S2 before you noticed the thumb pain? If the answer is several hours a day or more, you may just have a "repetitive stress injury" from overusing the left hand and thumb which has resulted in a straining of tissue and bone. If there is swelling it's likely you've strained the thumb/forefinger.
Just don't practice guitar for a week's time and ice the injury for 2 to 3 days. Common practice to help heal this type of injury is to ice the injury with an ice bag or freezer gel pack in a cloth. Even a bag of frozen peas will do the job. Then, slowly, use a moist, hot compress (facecloth soaked in steaming water) wrapped lengthwise over the thumb joint. Repeat every 3 hours. Do this for 3 days. If you thumb doesn't feel better after a week to 10 days, see a physician.
FTR, the doc would prescribe this treatment barring any other issues he or she might diagnose.
If perhaps the pain is severe enough that you can't use the thumb & forefinger, you may have jammed your thumb sometime overnight while sleeping (reached out and smacked the nightstand, or tossed your left arm and thumb over your head into the wall is a common cause). A jammed thumb usually works itself out in about 6 to 10 days. While not a guitar-related injury, it's a common injury while we work out our daily issues in our dreams.
Try the ice/heat treatment, and arrange for a doctor's appointment if the pain doesn't clear up in just over a week's time. Hopefully, nothing's broken in the thumb region that would require additional medical treatment.
