Penis is not made up of muscles like arms, biceps etc. It has blood vessels only. During whole day resting state, blood vessels will be empty and penis in smaller size. Its normal that penis stays short in resting state. When blood flow to penile vessels increase, penis gets erection. Ours penile erection depends on how much ours brain sends intense and full sexual signals to lower sexual organs - penis.
Next Steps
Don't take any medicine without proper consultation and supervision.
Health Tips
Consult me personally for further help . Take care !!