Lost my appetite

I am eighteen and have recently found I have completely lost my appetite. I have always been on the slim side, and am sitting on 48kg (which isn't too bad considering I'm only 156cm high). I have never found a problem with my weight (other than the usual insecurities that come with being a female) and have always eaten a horse for lunch and dinner. I can't eat anything before 10am without feeling physically ill, so I steer away from solid foods for breakfast. However, I am worried that I may lose weight or muscle density because I have no urge to eat. I do not usually drink very much, though because it is now summer I am succumbing to the constant need for fluids. I am very sensitive to temperature change and assume this is triggering my sudden lack of hunger. However, when I do eat, I become hungry, and am disappointed I asked for a small amount of food on my plate. I have seen no changes in my body either way, nor have I lost the ability to eat as much as I always have. I am just a little worried that this is unhealthy, and though I am combatting it with frequent small meals such as fruit or yoghurt, I want to be able to feel hungry again.