Guys thank you for all your replies! My thumb is always behind my first finger or in the middle of the first and second fingers. The problem is even more apparent now because now even basic scale exercises hurt my thumb. I personally think it's a streching problem. Maybe I should go see a doctor after all.
Yes see a doctor for any pain really.
Word of note, you should never seek to put your finger behind your thumb regardless.
Yes your thumb will find itself behind fingers, but that is more to do with angles, than technique.
In holding the thumb, or any part of your body for that matter, in an un-natural position will cause un-wanted tension.
Muscles use tension to work, they work in pair to always pull your body in positions, so un-wanted tension is a problem.
A definition of repetitive stain injury, is repeated motions under stress or tension.
So if you repeat the same motions in the same positions you are being repetitive, so vary the motion and how you use it.
One variation is to let the thumb go where it wants, let the finger position dictate the thumb position, not put the thumb in a position to dictate finger positions.
The same applies to elbows and wrists, we cannot always hold postures.
The longer any posture is held the more tension will develop...something to think about.
Good luck with it all.