I agree it pretty much depends on how serious the cause of the fall out is. In this relationship, it is quite common to have a misunderstanding. But if the misunderstanding is not caused by a blatant crossing the line, a disrespectful remark or action, patching things up is usually possible especially she is your long term regular. If it is a minor misunderstanding caused by the action on your part, a polite email or text to apologize woud usually do the strick, keeping in mind that she very much prefers a regular whom she is familiar with and likes.