I am 5'8" as well so wen I saw 98 i was like "wow!"
when I was sick before being diagnosed I got down to 105 lbs (maybe less...i stopped weiging myself since it only depressed me)
To gain weight I started eating more but I still felt sick. So here's what my doctor suggested:
eat small things... gluten-free crackers, granola, fruits, veggies...
eat every two to three hours depending on what you've eaten. For the first semester of my junior year it seemed like i always had food with me. it still hurt to eat though so it was a long progress. I recommend (if your system can handle it) lots of peanut butter, and other fatty calorie boosting foods. my mom bought me whole milk (yuck

) ad butter instead of margarines. things like that. there are lots of foods that can help you put on some healthy weight. remember to stay active though or the added weight won't be as pretty as you imagined...
nah im just kidding. just get healthy! best of luck to all who are having these problems.
it worked for me. i am now 5'8" and 123 lbs