That’s a very fine line and

That's a very fine line and would also depend on what you had filmed. What type of business was it? Was it a video company that employed you to specifically film things for them? If so, your work is their property. Or was it a non-film industry business where you randomly filmed things on your own similar to if you took random pictures for fun? Just to be safe, since you left on good terms, ask them if it's ok to use your work for your film reel. They shouldn't mind, but make sure to get it in writing! That's very important.