SMALL smalll crack on screen (just noticed)

a month or two ago i dropped my phone when i was getting out of the car and when it hit the ground the screen was cracked pretty bad, it still worked and u could still see the screen because it was on the part where you put your ear so there was no vision problem. I went to the apple store once and they told me it was going to be 250 bucks to replace it but they didnt have any in stock and told me to come back. I went back a few days later and told the guy what happend again, and this tiem the guy said that what happened was unfortunate and he said it shouldnt have cracked as bad as it did since i dropped it from a VERY low height maybe a foot, not even. that day i decided to buy a cover so he gave me the phone for free and just charged me for the cover. Now im looking at the screen and there is a smalll smallll crack there. Its very hard to see and you can see it only when the background is white in the part of the screen where the crack is. I can notice it because since its cracked theres a little rainbow color so i know its a crack and if i run my finger nail over it i can feel a slight bump. The size of the crack is maybe... a centimeter lol maybe a little bigger i dont know. Im a little picky about things like this because this phone is a big investment and i didnt drop it since i got hte new phone + it has the cover on so i have no idea how a crack this small got on the phone. Im thinking that when they gave me the phone since it was just a refurbished one that it already the crack. Should i go back to the store and request a new phone? or would it just be a waste of time?