Despearte for some help! How can I eat healthier?

I'm 22, almost 5'3" and about 98 pounds. Clearly, I'm thin. I'm worried that I'm damaging my health being this way but I don't know what to do? if I'm correct about my calorie intake---I'd say I come to 2000, but perhaps its more like 1800 on some days. Obviously other days I do go over 2000. I do exercise-but nothing crazy. 3 miles on the treadmill and/or the eliptical a few times a week.

Is it possible I'm malnourished/or under estimating my calorie content? Please take a look and tell me honestly how bad this is? How can I improve? I feel guilty or weird for feeling like I need to eat more--people tell me I don't eat much but I always figure that I'm small/petite so I just don't have to? It's not like I ever go hungry...The idea of gaining weight just seems...odd.

Im worried because I have thin hair as well-I have as long as I can remember but what if it's because I'm not eating the right things/enough? Honestly, i'm about to pull my hair out because I'm at a loss here as to what to do! It's so bad that I cry sometimes because I'm so worried about my health!

Today I ate:
-ham and cheese sandwich
-apple and guava
-2 chocolate bars, a cookie, a bit of cake. (Im not usually this bad-i try to eat healthier snacks...but today it was someone's birthday so...well there you go)
-rice & beef, chicken noodle soup, a small salad, and some chips (to be honest these were small portions...)
-lentils with sausage, toast (this was quite small too--i think?)