Meh, I was pretty light for quite a while. During your teen years is when you should start to notice some weight gain; I weighed about 99 lbs for a long time, because I was a bit of a late bloomer. I'm around 120 now which is still quite thin for my height, 5'8. How tall are you?
Basically it's normal, unless you feel unwell, have lost weight rapidly, or are constantly tired, then I wouldn't worry about your weight. You probably have a fast metabolism
(your body's ability to digest and recieve nutrients from food) so that's why you aren't gaining weight.
If you want to gain some weight, light exercise to build muscle tone
is good, you need to do muscle work though, such as weight training
, rather than cardio which will just burn fat. Ask a personal trainer to help you with this if you are interested. Otherwise, enjoy it while it lasts!