Fell the down e stairs 2 weeks ago had some shoulder pain. Now pain in shoulder upper arm and armpit area. There is a knot in armpit.?

Trauma. Something's changed in your shoulder.... Could be tendon or ligament strain or tear, muscle tear or sprain. Probably not dislocated but could have bone fracture. You should see primary doctor and consider x ray. Try heat and massage and anti inflammatories too.
Trauma . Sounds like you took a nice fall you may have sustained some shoulder injury ( rotator cuff ) the knot may be swollen tissue get it checked by an orthopedist.