I am a 20 year old female, I am roughly 5'9" - 5'10" tall, with a waist measurement of 25inches and a hip measurement of 34 inches. I was steadily between 130-145lbs through out high school, and went up to about 165lbs at my highest weight somewhere during my freshman year of college. I was never concerned about my weight, however in the past year or so, my weight has dropped drastically, and I now weigh about 115 lbs. I am not so much concerned with gaining weight as I am unsure why I have lost this weight, I have made no significant changes to my diet, nor have I become more physically active in the past year, and if anything I would say my stress level has decreased. Could someone please tell me what some possible reasons are for my 50 lb weight loss?? Is this something I should be concerned about?

To be honest, yes, I think you should be concerned. A loss of 50lb is almost half your body weight, and if there is no external cause, then you should go to your doctor and get it thoroughly checked out. Something like an over-active thyroid is an obvious explanation, but it could also be something like diabetes or a lot of other things.