When you start to question whether he/she has a crush on you , you may be secretly in love with him/her. That may not be as intense as love. It can be a romantic feeling in different extents. The following is some English expression to state the beginning of the romantic relationship.

1) Have a crush on sb. : romantically attracted to someone, but the other may not feel the same as you
I think I am having a crush on a girl at my university. She is so elegant and sweet. I can’t stop thinking about her.

2) Flirt with sb. : do or say something to show romantic attraction
I don’t know if I am truly in love with him, so I am flirting with him for now.

3) Have a chemistry with sb. : both of you feel romantic attraction towards one another
I couldn’t believe that I had chemistry with someone whom I just met for one day. We have talked a lot about relationships and life goals. We had many things in common.

4) Love at first sight : fall in love with someone when you first met them
Do you believe in love at first sight? Are people only fond of the appearance of someone?

5) Be infatuated with sb. : deeply attracted to someone romantically
When you are infatuated with someone, you will easily overlook their weaknesses that you may not be able to stand later.

There are differences in the intensity of romantic attraction. So, what’s the best phrase to describe your feeling?