I NEED TO GAIN WEIGHT

Okay I've always been quite skinny! But I had a cute butt to go along with it and thighs that didn't touch.. I was meaty but skinny still. My waste has always been tiny.. I've never had large boobs.. but in the past year or some odd months.. I have lost weight and I don't know why or how. I've been to the doc for something else and I have a couple points off on my thyroid which could be messing me up a little but I've lost probably like 10 or 15 pounds which doesn't sound like much, but when your skinny it is! I want to start working out.. to tone my butt and lift it again.. but I don't want to do that until I start gaining some weight. I now have a thigh gap! And I don't want one. My rings are basically falling off of my fingers.. my shoes are a little to big and my bras are needing more padding.. and my skirts are getting a little to lose for what I like, because I already normally wear a 4 or 2.. if anyone has ever experienced this your comments and tips would be great! Or just anyone that can give me some tips to gaining some weight.. that would be nice too! Also... I DO eat y'all!! I eat any and every thing! I don't drink water a lot.. I'm constantly eating junk food too.. BUT sometimes when it's inconvienant or I'm busy I won't eat when I'm hungry and eventually I forget I'm hungry and won't even eat. And I skip break fast and lunch a lot and snack throughout the day which I know isn't good. Anyways, just any tips would be much appreciated! Thank you