go see a orthopedic hand doctor. call around and find one that has x-ray machines in the office, it'll save you time. i'd avoid a general practice doctor unless for force to seen one for a referral. good luck
Hello!
Get an X-ray done to rule out fractures of you thumb.
Take rest from karate for couple of days, do not exert your thumb overtly.
Apply ice packs and take NSAIDs intermittently.
Take care!