Probable cause doesn't enter into the search here. They appear to have plenty of probable cause for the arrest. Once you were being arrested, the officers are allowed to search you and your immediate possessions to make sure you are not carrying weapons that could be a danger to them. Just what the nature of the search of your bag would determine whether that search was properly conducted and what if any evidence obtained by that search would be excluded in court. However, prior to any of the searches, you'd already confessed and provided evidence of your guilt, so any subsequent searches, even if improperly conducted, won't change that.