welcome,
consult your family doctor for physical examination.
1-mastrubation-no harm ,no relation with weight &
back pain,you can continue, consult counselor,
2-cause of weight lose may be sugar,
thyroid,digestive problem,
get
blood test for thyroid profile,HbA1C,sugar,hemogram with esr,
urinalysis,
3-for back pain x-ray /mri spine,
treatment depend on cause,
avoid smoking,alcohol,cola,coffee,junk foods,
take fruit juices,banana + milk shake several times a day,full cream milk & milk products,indian parantha,potato,butter,
take care,