There's nothing wrong with having a crush on more than one person.
But if you're going out with someone you like and get along with then it would be kinda stupid to throw that out of the window just because a guy keeps looking at you in class.
It's weird how the human mind works
Have you ever noticed that if someone tells you that a certain boy has a crush on you, he somehow becomes more cute than he was before...?
Everyone likes to be admired. It's a feel good factor. When someone's attention makes you feel good, you naturally find them more attractive
This might be what is happening to you.
If Alex is new, you don't know him too well just yet - but he's different; not just one of the boys who has been in your class for years.
You don't know if he's a flirt who just likes to encourage girls to look at him either