Hello der,
Well here you are having two concerns.Let me look at them one after the other.Your first concern seems to be the weakness you feel in your arm.You have to understand that our muscles have a set reserve of energy which when you do some activity ; as in your case masturbation,that reserve is used up.This results in fatigue of the muscle and the muscle cannot perform its function till its energy reserves are restored.This manifests as weakness.Here you have mentioned that you masturbate daily but dnt mention the frequency per day.If you have recently increased the frequency of masturbation per day then due to the above reason you may feel your muscles to b weak and tired as you are not giving enough time to replenish their energy reserve.If you havent increased your frequency then the weakness may b due to overall weakness in your body or may be due to a new physical activity that you might have taken up recently.
My advice to you is to give rest to you muscles .Take multivitamins and eat healthy.
As for your second concern about erection.Well masturbating evryday does have its effect on erection. People who masturbate regularly find it difficult to either have an erection or to maintain it for long.This results in performance anxiety which in turn causes problems in maintenance of erection and then the person goes back to masturbate and the cycle goes on.For a healthy sex life my advice would be to try stopping to masturbate everyday.U can slowly decrease it progressively to twice or thrice a month.This will surely enhance ur experience when you have sex the next time.
Hope your doubts were cleared.Thank you.