masturbation problems,,,, (back pain), hairfall

i'm age 21.. i do jerking twice or thrice if i get the mood in about 20 min,, i do the (pillow,&sandwich cover) technique,, that really gives me the feeling of inserting, but after that i get the back pain,, and i observed hair fall,, my penis size is 5.5 inches,,
i do like this twice or thrice in a week,( totally 6 times aweek),,,
is this causing me back pain problems,, or what,, i'm a healthy person