My 17 yr old step daughter had a similar thing. And her's was also around exam time. She went to see a specialist who did all sorts of tests including nerve conduction tests and the result was.... it was the way she held her pen when writing....she ended up with a kind of RSI ( repetative strain injury). She is now seeing a hand specialist who is helping her retrain how to write properly.
I also suggest you see a doctor. There is nothing to be afraid of. the doctor will check your thumb out and give you good advice :)
Thanks guys hopefully I will :)