numb pain in tricep

hi doctor, I have this habit after several days of outside bouldering to get this pain in my left tricep - more accurately on the top of the 3 headed (?) triceps muscles on the side of the arm. From there it kind of radiates to front shoulder and and so on. Really left hand all numb and had to stop climbing. After a day it still aches and feels more or less powerless, the arm. I am sure resting and strecthing will do the trick but I´d be more interested to know perhaps what causes this, because this tends to go like this always after 3-4 days bouldering. I do have a really sore muscle spot on left side of upper back - could this be something to do with muscle nerves being jammed somehow..?    thank you in advance and wishing nice spring climbing