Hi there.. backpain knee pain, al these are because of muscle strain due to exhaustion after masturbation.. it could be because of anemia, vitamin d or other vitamin deficiency causing you weakness and pain after masturbation or from no physical activity to sudden exertion during mastrubating could be the reason even.. have more nutritious foods rich in proteins, have milk, egg and green leafy vegetables.. you can have multivitamin supplements even. Do regular exercises as it helps you to increase your stamina..hope this helped you..