You could have then had the covering agent write the offer for you and you would not have had to contact the listing agent. If the offer had been written and not just verbal, there would have been discussions about representation and you would have signed paperwork acknowledging that. From the way it is described in your question, the behavior of the listing agent was very unprofessional as well.
If the property is still available, contact the Broker/Manager ask to be represented by a Buyer's Agent, sign a Buyer/Broker Relationship Agreement with them, THEN have that agent write your offer and submit it to the listing agent.
I think the listing agent here could be in quite some trouble. If you have already signed papers with your agent, and the listing agent called you directly after knowing this......WOW.
If what you say is correct:
I would tell your agent what happened and ask that the other agent be reported to the board.
NO OTHER AGENT has the right to speak to my client if I have them under contract for any reason without my permission. If they felt the need to yell at you, they would be doing it through me.