Will Discontinuing The Use Of Retainers For A Short Duration Cause Problems?

Hi! I am 18, and very self-conscious about my teeth. I have always had good dental hygiene, but recently I have had some problems. I have been home from college for Christmas, but forgot my retainer (I got my braces off two years ago) in my dorm at school, so I haven't worn it for two weeks and I can tell that my teeth are shifting a little (I wore it every night until now). If I start wearing it again once I get back to school, will my teeth move back?
Also, I love brushing and flossing two or three times a day, so I think I have pretty good dental health. BUT, I have noticed a little vertical groove in my top front tooth that comes down from the gum line about a quarter of the length of the tooth. It's small, and I can feel it if I run my tongue across my teeth. It is not discolored, and it's sensitive to cold, but my teeth have been sensitive since I was little, so that's nothing new. So I have no idea what is causing this little groove, but I'm worried it could be something serious (which would be weird, since I like to brush). Please help!!