Why sometimes i feel pregnant then i dont?

I know iam pregnant big stomach and all symtoms and all it moves lol etc but why sometimes I feel I'm not then deep down I know iam like I think its cause school coming up iam 17 and I wanted to look pretty for school but I have a stomach sometimes I don't wanna eat but if I ddont I feel sick even if iam not that hungry and sometimes I put sensa on my food which is stuff you put on your food so you won't eat as much help please thanks