Take it back to the shop that did the repair and tell them that it is leaking oil - have them determine where the leak is and why since they just did a motor R&R for you they should accept some form of liability for it (unless they specified that there was no warranty for parts/labor). What could happen if you keep driving it like that? Well worst case scenario is you lose oil pressure to the point that it causes severe engine damage that requires replacement again. I realize that you say it's a small leak but that is typically how most oil leaks start, small then get more and more substantial. As 'choxor' said, not your responsibility to diagnose their mistake/oversight.
"! First rule of Italian driving, that which is in your mirror is not important !"