At the age of 13 I got my period and never have been regular. I am now about to turn 20 and still irregular.I sometimes go about 3-4 months without getting my period. I am not sexually active nor never have been.
yes, christie is right... my other suspicion is, have you started a rigorous excersize routine. i have had three friends who began to excersize like crazy to lose weight and stopped their periods entirely until they relaxed the excersize a little - unfortunately we are all different and some women are very sensitive to changes in diet, excersize and hormone...
even if you think you may have the answer to your problem with diet/excersize, it is a good idea to go see an obgyn and report this and ask questions - i would have your thyroid checked, as it regulates hormonal changes, so if something has gone ary there, you can take meds to regain normal function...
also, you may want to ask about 1. anovulation (which can be treated most of the time) and 2. polycystic ovarian syndrom (which is not at all likely in your case from what you have discribed) But honestly hon... an obgyn is never a bad thing to have to turn to so that you can treat your issues, become aware and put your mind at ease... also, if there has been major stress in your life, you will want to try to destress as well - good luck.
If you are doing some sort of extreme dieting, it may take until you resume a normal food intake for your period to return. You didn't say how long it took to lose that amount of weight. Be sure you are not going under 1200 cals/day. If you are extremely low carbing it, this alone can cause menstrual irregularities.
If you are eating in a healthy manner and your periods have suddenly changed this drastically, you should probably go to the doctor and get things checked out. There are some gyn conditions that can be responsible, it is not always diet-related.