Need advice!!!!

does anyone ever get that feeling, like when they're about to go into a relationship or things are getting serious and all of a sudden younfeel like it's not a good idea and you feel sick to your stomach? Because there's this guy and like I really really like him and he likes me but the thing is is that I can enjoy being single but at the same time I want a relationship. I don't know what to do can someone help?? and how would I tell him that I don't want anything serious. Is that a good idea to say