You first mentioned that your initial offer was rejected. Then you mentioned that you were informed verbally that your offer is in backup status giving the impression it will be submited with all other offers received on Monday.
If your inital offer was completely rejected and will not be re-submitted to the Seller'(s) on Monday then you may choose another Agent and Brokerage to represent you in making a new offer. You will have no obligation to your past Agent and Brokerage in this case because your initial offer is no longer being considered and you have no Exclusive Buyer Agent contract with your past Agent. However, I must stress again your former offer cannot be included in that group of offers to be submitted on Monday. Good luck!
You could always go to another brokerage, but be sure to explain the situation to them and let them work out something with the original brokerage. This may be a bit harder to work out, but you will get a clean break from the previous brokerage.