"Love at first sight" is when you see someone, and immediately feel like you love them. It sounds very romantic, doesn't it?
A: When did you fall in love with him?
B: For me, it was love at first sight.
When you use the phrase "love at first sight", you don't need to say "I felt..." Just say "it was".
By the way, you can also use this expression to talk about things that you quickly fell in love with, as well as people:
As soon as I saw it in the lot at the car dealership, it was love at first sight.