Why do i feel weird with guys? help?

There is a guy who likes me he is really nice but not that good looking. I like him but get a strange felling around him not a good one its like I don’t want to get close. I been out with guys before but the same thing happens so I end up dumping them. Is this normal should I go out with him or not. this only happens when i'm with someone who likes me or i like them not all guys. i'm also very shy so its hard to let the feelings p. but when i'm not with someone i want a bf but when i'm with them i don't want one. help me!