Why do I always feel sick to my stomach after kissing someone?

Depends on.. How many people are you kissing! if many people simultaneously, then you area "player" and feel bad about yourself, ie. Not really caring about the peole you are engaging in lip-sex. You may be frightened of catching something. Many reasons. But this should not be a problem in a trusting caring relationship. If it is, you then need to talk with a therapist!