Egg-sized lump on forearm 1.5" from wrist, i think on the tendon extending from the thumb. very painful, what should i do?

Depends: Probable Ganglion cyst. Go to a Hand doctor for evaluation and treatment. Hope the pain resolves soon.

Ouch!: A lump like that could be a rupture in the muscle or in the tendon, or a hematoma. Depending on one's insurance restrictions, or the availability of specialists in the area, one can call her primary care doctor for advice, see a general orthopedic doctor, see an orthopedic-hand specialist, or go to the ER where they can evaluate and call the orthopedic doctor on call for the ER today if needed.