More tension means less blood flow, and the cycle continues. Another by-product of the lack of blood flow is tingling and numbness from your nerves. They need blood too. Anyway, when you're typing too much, you're never really giving a change for the blood to get back where it belongs, because your muscles never relax enough to let the blood through. Stress, poor posture, and poor ergonomics, only make things worse.
Acute injuries are severely painful and noticeable.
Tenosynovitis - an inflammation of the tendon sheath. As a result, the tendon sheath thickens, gets inflamed, and you've got your problem. Tendonitis - an inflammation of a tendon. Repeated tensing of a tendon can cause inflammation. Eventually, the fibers of the tendon start separating, and can even break, leaving behind debris which induces more friction, more swelling, and more pain.