Is it normal i'm afraid to gain weight in a way...

I think you're wrong about what the lower bound for healthy weight for 5'9" is. 140 is fine, but based on any BMI scale, the lower bound is 125. I thought that your quote of 140 sounded high to me for a woman to be the minimum, so I looked it up and it is.

imagine yourself with sexy curves instead of a bean pole body.

Weight means nothing. Bodyfat percentage is the what really matters. You should be exercising. Lifting weights doing cardio...I'm seen plenty of skinny fat men and women...by the numbers...they are not overweight but they have guts.

Exercise. Build your body.

I'm 5'9 as well and I weigh 120 pounds people tell me I need to gain weight too even though i do have curves and im not "pole thin" but as long as your happy with how you look and your not starving yourself dont worry about! some bodies just can't gain weight because of metabolism. so just enjoy your body.

If you're not hungry, tired or run down then you should just continue heating what you like when you like. You obviously don't need to count calories etc as you are so light.

Eat what you like and enjoy - you can afford to put on a few lbs.

You're only 18. 18 year olds tend to be skinny. Enjoy it while it lasts. Definitely don't make a point to gain weight. When you get older, your metabolism could easily go to shit anyway and you'll long for these days. When I was 18, I think I was around 105 and 5'4". People told me I was too skinny. I wish I had that problem 19 years later. Now I'm 138 and want to lose weight.

You sound fine to me, don't listen to them.
If your doctor tells you that you need to gain, that's one thing.
Some people have faster metabolisms and small appetites, that's normal!

If you have always been like that I don't see a problem here unless a doctor tells you otherwise. I'm 5'4 125 and people still tell me I need to eat more and that I'm too skinny. It's because they're jealous. If they haven't known you very long then maybe they're worried about you but just tell them you've always been like that. I'd only worry about someone I SEE has lost a lot of weight.

Isn't jennifer nicole lee like 5'9" and 130? she's a lean sexy bitch.

You should probably just aim for another 10 lbs and see how it feels (if gaining weight is what you want).

Well, nobody wants to gain 23 pounds no matter what the current weight. I say, if a doctor tells you you're fine, you're fine. If it's too much pressure brought on by others, you could try gaining 10, but if a doctor has told you you're fine, you should really just tell people they can f*ck off. If you want to be a 'bean-pole' as others may put it, as long as it's healthy, it's your business.