Why is my thumb swollen for no reason ?

Several things can..

Extremity-upper-hand
...cause a swollen thumb. if you do not have a history of trauma, this list includes stenosing tenosynovits (trigger thumb), tendonitis, degenerative arthritis, gouty arthritis, rheumatologic conditions, and infection. if the swelling last for more than a few days, i strongly urge you to see a board certified hand surgeon for a comprehensive evaluation.
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Arthritis Tendonitis Trigger thumb Gouty arthritis Hand Thumb Hand surgeon Infection Fingers

Lots of reasons

Thumb
Arthritis, trigger finger, carpal tunnel syndrome are but a few of the possible reasons. Another reason would be infection and you don't want to ignore this. See your doc.
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Carpal tunnel syndrome Arthritis Trigger finger Infection Thumb Fingers