You do not need to lose more weight. A weight of only 135 pounds is actually at the low end of the range for someone as tall as you are. A healthy range for someone 5 feet 9 inches tall is 130 to 160 pounds, depending on bone structure, muscle mass, body type, etc.
It's common for people who have lost a lot of weight to have areas of "flab" left over. You can help to tone and tighten this "flab" by doing specific exercises that focus on the various abdominal muscles. Losing more weight will not take care of it. A physical therapist or a trainer should be able to help you determine what types of exercise would be best.
I realize that the images presented to girls in our culture may make you think you should have an absolutely flat stomach with no visible fat on your body, but that is not a realistic nor even a healthy way to think. Some young women get caught up in thinking that they should do anything possible to look thin, and this can unfortunately lead to eating disorders, such as anorexia nervosa and bulimia.
Exercising regularly (four to five times per week) is probably the most important thing that you can do to maintain a healthy weight that you will feel comfortable with.
You should also look at your eating habits and make sure that your diet provides you with the proper nutrition while not giving you extra calories that your body doesn't need. Make sure you get good amounts of calcium, protein, B vitamins, folate, and iron, which are very important at your age.