Why cant i eat like i use to?

i lost alot of weight well i think the weigh machine is broken but i lost at least 10-20 pounds i kant eat as much as i use to at all like lets say a mc donalds cheese burger i kan only eat maybe half and i will feel stuffed but it keeps getting worst like the amount of food i kan eat is getting less and less im getting scared b kauz this has never happened to me b 4 oh yea im about to be 17 in less than a month but rite now im 16 5"4 an da scale is reading 99 pounds den it read 110 i think 110 is more like it tho but it use to b 138 or so like 3 weeks ago am i ok?? kauz i kant eat an am loosing wieght and i kan tell i have lost wieght b kauz my stomach dnt stick out no more lol but yea i dnt kno wats going on