If you and the prosecutor came up with a plea agreement, this agreement still needs to be placed on the record with the judge. It is likely that they still have your signed agreement but are just giving you a court date to place it on the record. You should not have left the court until it was placed on the record the first time, but now unfortunately you will have to go back and complete the agreement. If they do not have the plea agreement on file, it is possible you will have to bargain with the prosecutor again.
You most likely struck a deal with the prosecutor and never placed it on the record in front of the judge. If you did go in front of the judge, there may have been a record keeping error. You should be able to resolve it at the next court appearance.
The proposed disposition needs to be heard by the judge and placed on the court's record. The judge will confirm that you are willing to plead guilty to the amended [downgraded] offense and voluntarily waive your right to an attorney and trial on the original charge. The judge will then sentence you [fines] and you must pay same at the violations window.
When you made the plea with the prosecutor he or she should have given you a copy of your plea agreement. If you wish to maintain your plea, I would suggest simply showing it to the prosecutor the next time you go to court, especially since in some towns there may be more than one prosecutor. I would see no reason why they wouldn't honor the agreement. Of course, you could also disregard the plea and see if you can get a no points plea, but that depends upon the circumstances around the ticket, and also your prior driving history. But yes, you must go in front of the judge to get the plea on the record.