When I turned 18, I bought a pack of cigarettes just to try it once. I only smoked half the pack before tossing it and haven't smoked since then. Well, I'm 21 now and I'm suddenly getting a craving to smoke a cigarette out of nowhere. I don't plan to give in to this craving, but I'm just wondering if it's all in my head or if it's possible that the addicting nicotine affected me? Just wondering.

Unlikely that it is nicotine after this amount of time, but smoking can affect people in different ways. Once you have smoked you will always have feelings for the action and effects it gives.
I have recently quit after 20+ years and find that mint sweets help if I get a craving..

It is all in your head. You did this at a mild stone date ( 18th) birthday. So now on another important birthdate, you have mentally related smoking to an important date.