How do you know that you have anorexia?

well I guess it all started two years ago . now I am almost 15 . when I was 12 I was very chubby, nothing like my parents and brothers who were really skinny . it didnt bother me at all and when I started doing sports I lost a lot of weight and finally had a ‘nice’ body . it was in the summer in the yr 2005 when I went to see my ‘boy friends’ family [[who I havent seen for like a year at that time]] and his mom was very surprised by the way I looked . later that night I heard her speaking to someone in the living room . I dont eavesdrop, it was an accident that I overheard while I was going past the room, and she was saying that she was concerned about me, that I have lost so much weight so quickly and she was scared if I didnt have anorexy. as a child who didnt have a clue what anorexy was, I totally forgot about it . but then about a year later I looked anorexy up on internet and found out all the symptoms and what it actually was . at that time I laughed to myself, thinking how stupid my bfs mom was , as I LOVED to eat. but then I dont know why, I started to eat less and less, everytime I looked into the mirror I shrugged . Now its been about a year and all I eat throughout the whole day is : an apple if anything at all for breakfast, nothing for lunch and very little for supper . Every day I get from school I straight away go and check what my weight is and now I weight less then I did when I was 11. at the age of 11 I weighet 55 kg and was 145cm tall, now I weight 49kg and I am 160cm tall. im not skinny but I know I am not fat. but I feel very fat and every time I look into a mirror I just see fat fat fat its like I am aware that im being stupid its just that I cant stop !! I mean every time I eat something [[even like a tiny peace of bread]] I feel guilty and I refuse to eat anything else .

well thats not all . recently my best friend stopped talking to me . she knows that I dont eat and she tried her best for the past months to make me . but few weeks ago we got into a fight about it and she said that I was selfish and since then she hasnt spoke to me .. I cried a lot because I know she’s right , it’s just I cant get rid of that feeling every time I eat , it just makes me depressed, weak and tired .

could this be the start of anorexy ? or is it just what happens to teenagers when they;re very critical about the way they look ? please I NEED AS MUCH ADVICE AS POSSIBLE thank you x