Knot in my throwing arm.?

I have had this re-occurring pain that is brought upon by a knot in my upper bicep/lower shoulder after I pitch. It becomes incredibly tight and is uncomfortable to throw with this pain. My arm is sore and lacks motion the following days after I throw a bullpen. I have tried icing it, heating it, and even massaging it a little as well. If anyone has any ideas of what it may be, I truly appreciate it. Thank you.