theres a tendon that runs down the back of your arm, you dont wana hit that! lol that might have caused the jump.
either way dont freak out as those happen from time to time as does occasionally hitting a cray painful spot. i think too often ppl say i hit a nerve or vein or some other scary idea cus a shot was more feely then usual.
in tris, quads, and lats i have touched a something that absolutely took my breath away, lol like spectacular pain for an instant. i always push the pin in supper slow so have learned to not really move or stop just pause adjust and keep going. knowing that tomorrow it would be sore and swollen.
the blue and red area of that are good all day but they leave out room at the top which is really where i feel the best spot is, like right under where your delt goes in towards the tri, like two inches higher then right where the blue and red meet in the pic. if you do the blue too low where it shows it gives you a weird bump in the tri, too far forward too. but for the side tri hit where it ties into the delt then the long to get the best idea stand facing a mirror flex your bicep and find the lowest spot on your arm. pin there, you can do it just like that or hang your arm by your side and generally can hit it the same. back there, there is also a super ouchy spot, find and avoid. lol
i used to mark my skin with a pen or marker, like a point where the inj needs to go then a biger line above and line below. clean in between the lines and use the point as a reference here to inject.
if you inject some volume weather it be synthol or aas done right like for your frame, muscle tie ins and bellies you can add real pop, that i feel you dont get in other ways.
my delts for example since not injecting them for years are much less round like out wards. but over all too much different. doing this well is worth the little pain it takes.