text: why does your arm feel sore after getting a jab? i just took a hepatitis a and b jab and my arm is sore! why is that? because of the fluid they put in.    some shots have more fluid and some of the fluids are thicker than the others.  the more and thicker the fluid, the more it displaces muscle tissue in your are.  that is what causes the pain to last.  the needle stick itself doesn't hurt for long. the fluid it self can also cause stinging.  some injections dissolve in fat and some in muscle.  the bigger the muscle the less it hurts