GaiN Weight? Too skinnyyyy?

I am 5'8.5" and I weight 120 pound. It sounds like I wouldn't be too skinny but I am. I wear a size 0 jean and mywaist is 25" and my hips are 33". I have really nboney shoulders and my chest is really bony too (like above my you can see the little lines fromI my bones ) pol are always asking me if I'm anorexic and what not and it's really annoying. I eat like A TON OF FOOD an it's mostly healthy like I eattones of protein everyday I have protein shakes etc. Ialso eat a lot ofjunk food on the weekends too. I can outeat most of my friends who are l bigger rthen me. I also started working out for 30 minutes every day because itI was supposed to help me fill put more and nothing is changing!! My arms look a tiny buy mor toned but that is all that's happnig I cannot gain weight and it is so annoying! I'm tired of being bony! Please help me! How do I gain weight?! And please no one say eat junk all the time because not only does that not work but i dont just want to gain fat and have nasty cellulite I just want to not be so darn bony and fill out a little while still keeping my fit body. Help!