Ladies Only: Lady problem

So your doctor probably told you that you're not getting an LH surge, meaning no ovulation and no period basically. At 110 pounds, you're probably overdoing it in the gym and not compensating with enough healthy food, especially healthy fats (think nuts, avocado, olive oil. salmon).
I workout a ton and still manage to have regular periods (training for sports), but that being said everyone's body responds to the stress of working out differently, so you might just have a lower threshold for hormonal disruption from working out.

missing periods 4 months in a row has you meeting criteria for secondary amenorrhea. I agree with the previous poster who said you should look up the female athlete triad, its actually a quatrad now, they are finding that the triad is also associated with coronary artery disease (as well as osteoporosis). Things we want to avoid:( Estrogen has a protective effect both on your blood vessels and your bone density. also as a side note, pregnancy could be another cause of missed periods.

Im sorry about your last post where you said you are dissatisfied with your body:( From your profile pic, you look beautiful. You probably want to focus a bit less on reduced calories and a bit more on weights for toning, muscle mass would make you look leaner and eating a bit more will make you feel better, focus better at school and regularize your hormones, plus it builds bone density. Cherish the body you've got and work to make it as strong and healthy as it can be:)

I know my sister didn't get her period because she didn't have enough body fat, and that could well be your problem as well. My sister was an extreme athlete and had some eating disorders, so it all added up. Make sure you have enough healthy fat in your diet and on your body! But if you prefer to be athletic to that degree, you may need to accept that you won't be getting your period.

Don't fear the pill. I have been on it since I was 17 (before I was sexually active) to regulate my periods. They have improved leaps and bounds. I take LoEstrin Fe (or whatever it is called these days) and actually dropped five pounds on it because the hormones are so low.

Not to be a Debbie downer, however, you need to weigh ALL of your options and get 2nd and 3rd opinions with regards to contraceptives. Yes, they have benefits, however, they have drawbacks as well. I had been on the pill off (when I wanted to get pregnant) and on (heavy menses, as well as not wanting to become pregnant). The drawback was that after awhile I developed breast cancer that was estrogen fed. Meaning, I have an overload of estrogen that the cancer fed off of. This is the short version and I'm sharing so that you weigh ALL of your options. Speak with a gyn and if you are still uncomfortable, get another opinion. And before anyone goes on a tangent, I AM NOT SAYING THAT EVERYONE USING CONTRACEPTIVES WILL DEVELOP breast cancer. Good luck on whatever YOU ultimately decide to do.