Is it normal that i like food but thinking about it makes me feel sick

So, I like eating. My favourite dish is sashimi. (It's like sliced salmon). But I can't eat a lot, whenever I think about food, my stomach feels sick, and if someone forces me to eat "a little more" I end up feeling nauseous, and my stomach feels like crap. I mean, my dad makes me eat a slice of bread, an egg and half a glass of milk for breakfast... and the whole process is really hard for me because it's just way too much food for me to handle. And when I'm hungry, (usually around 4-5 after not eating anything but breakfast) I just wait for it to pass because the thought of eating makes me sick. I'm not anorexic, I don't use laxatives or keep a food diary or anything. My BMI is 17.5, which isn't that low...(I think.) But I know people say it's underweight.