Today I was out with my group and the guy I liked came to talk to the guys I was with.
As I saw him I began to breath heavy and I felt really sick.I saw the guy I like hug a girl and I don't know if he kissed her too and then my friend talked to me and I felt like I waked up from a nightmare and my friend told me that I was breathing really heavy and that I was really white and when I told her what I saw she told me that he didn't do it.
Can it be I was imagining because I'm obsessed with him?
Can it be because I started a "diet" today and all I ate 2 small portions?
And did it ever happen to you?

If you can answer me, because I'm worried.

It sounds like maybe you were really nervous about seeing him because you like him. If you saw him be affectionate with another girl, maybe you were just really disappointed. You could have been a little surprised to see him. Also, if you aren't eating enough while dieting, it can make you feel faint. When you diet, make sure you are still eating 3 balanced meals a day, just choose low-cal foods and exercise. If you don't eat enough food, the weight you lose can be muscle instead of fat. It sounds like you had a little adrenaline rush when you saw this guy. I'm sure lots of people have had butterflies in their stomach when they saw someone they like, I know I have. If you feel faint or sick anymore, I would see a doctor just to rule out a medical problem. But, if this was the only time it's ever happened, I would say it's most likely just adrenaline.