I think 100 pounds for someone 5'5" is definitely a bit on the "too thin" side. I would recommend that we hook up a machine, and suck some of the fat out of me and put it in you!
If anyone else is too thin, I can certainly help you out.
If you find it hard to gain weight, I would recommend eating more balanced meals. A hershey bar and a banana isn't really that great.
Try eating a larger breakfast, say, oatmeal with cream and some sugar.........use whole milks, eat lots of nuts. They're high in fat but it's a good kind of fat. Avacados are very high in fat, but it's very nutritious. Use olive oils when you cook.
You may want to get your doctor to check your thyroid. You could have an overactive one which could be causing the problem.