Help with amenorrhea?

I am 18 years old. I am 5 ft 1. My periods have always been regular. However, this past year has been crazy for my body. I had a physical done September of last year ,and I was about 120lbs.Later that year, around March I visited the doctor for a check up, and I was 113lbs. I wasn’t trying to loose weight. It just... show more I am 18 years old. I am 5 ft 1. My periods have always been regular. However, this past year has been crazy for my body. I had a physical done September of last year ,and I was about 120lbs.Later that year, around March I visited the doctor for a check up, and I was 113lbs. I wasn’t trying to loose weight. It just happened. My periods were regular until July when they stopped. Since starting college this past fall ( I have not been stressed), I have started working out lightly 3-5 times a week. I started counting my calories. I try to stay around 1,600 calories. I now weigh 107lbs. I have not had my period since July. I may have amenorrhea. I don’t want to tell my mom, see a doctor, or gain weight because I feel the best that I have ever felt. I can’t see a doctor without telling my momWhat do I do? Sorry this is so long just looking for answers.