Weight loss

Patient: Hello,i am a 17 year old female, i cant explain what is happening to me. i dont no whether to go to the doctors or just to leave it.for years i have been at the weight of 8 stone, i have never gained or lost weight and in the past week i have went from 8 stone to 7.8 stone. i have weighed myself from the 3rd till the 10th and in that time i went from 8-7.8. on the 3rdi was 8.1 stone, on the 8th i was 7.13 and today i am 7.8. the only reason i started weighing myself is because for about a month now i have been realising that when i am eating a meal i seem to not be able to finish it and i can only eat certain portions of food before i feel full but i have always been like this, well thats what i think anyway. but my weight has never moved and for some reason i am dropping weight drasticly. what do i do?