I have not had any orders on my gig yet, just had 75 impression and 3 clicks. But I was thinking I have a portfolio that showcases my designs, should someone ask to see my work am I allowed to give them the URL of my designs?
You can only use links allowed in Fiverr TOS (help section).
As @marinapomorac says you need to only use approved outbound links.
You really should have a Portfolio easily available so if not able to manage that in the few slots provided, offer a way to let people (who care - and they are the only ones you want to work with) see you in a space that you have some control over. how a person arranges their work says at least as much about them as the works themselves.
I mostly use YouTube and that can include Playlists:
Mix Walkthrough - YouTube