Can being underweight make you have constant neck, shoulder and back pain? and also insomnia? i dread nighttime cos it takes me 3-4hrs to get to sleep.

NO: Underweight should not cause those problems. There is something else wrong and you need a doc to evaluate you thoroughly.

Not by itself: However, the cause of your underweight may underlie all your other symptoms. Depression can cause lack of appetite and insomnia -- and many depressed people have generalized aches and pains. Please get a full health workup with your physician, and get to the bottom of all this. Help is available -- if depression, there are good treatments. Be well.