Did you sign a Buyer Agency agreement? If you did...then yes, you have a legal agreement with your agent. If you look at a house with another agent, and end up buying that house...your agent could sue that agent for a commission.
If you don't have any written agreement, you can contact whatever agent you'd like. But...I'd be very careful about going directly to the listing agent. They are working for the seller, first and foremost. Some states allow an agent to act as a "Transaction Broker" and work with both buyer and seller. But...it's a very fine line, and it might be difficult for them to treat you 100% fairly, if they had a relationship with the seller first. Make sense?