No... you're not obligated to use the listing agent (unless, as others have pointed out, they got you to sign an exclusive buyer's agreement when they showed it to you).
This is one of those instances where you, as the consumer, are allowed to use whomever you want. If you want to hire a new buyer's agent, and bring them to the property... you are allowed to use them.
Now... it does complicate things. Procuring cause, might mean that the listing agent is entitled to a portion of any commission that is due the buyer's agent. But that's not a problem you should worry about... tell your buyer's agent the situation... and let them work that out together. What happens regarding the commission, isn't something that should burden you.
Remember a buyers agent is exclusive to you and you only. And it does not cost you a thing to have a buyers agent. So why not right?