There are complicated rules about pension contributions, particularly for private pensions. It could be that making the contributions after you left breaches the rules (especially if you were not working or were working and making maximum contributions to another scheme), but only an expert can tell you.
I suggest you take this a step at a time. Tell your ex-employer what has happened and see how they react. If you are not happy with what they say, contact the pension provider and ask their advice.