I think you are too thin to have normal periods. If you want to trim your waist you should concentrate on abdominal exercises but stop dieting.
The government website said that a female your height, weight and age has a BMI of 16.6. At ninety pounds you would have a BMI of 17.0 and move into a healthy level. From this weight to 130, you would still be in the healthy range. 135 pounds would be considered overweight with a BMI of 25.5.
http://apps.nccd.cdc.gov/dnpabmi/Result.as px?&dob=6/28/1993&dom=6/29/2009&age=192&ht =61&wt=135&gender=2&method=0&inchtext=0&wt text=0
Body mass index can be tricky at times, however. Very tall people or those with a lot of muscle mass can sometimes register as overweight on a BMI calculator. But I agree with its result that you are underweight.

you dont need to loose any weight you need to do chrunchies. for example lye on the floor and lift your shoulders up and your legs up, and pause in that position (like a dead cockeroch on it's back). you will feel a tightness in your stomach. try to do 10 of these every night.