Contacted out of the blue to say I’ve been paying the wrong amount

Agreement with the CSA for my husband to pay £25 per week which started in jan 09. Paying that money directly to CSA since then every week without fail. Not heard from CSA until today. Letter states we should have been paying £22 per week.

The letter does not state that we owe this money but I find it strange that they contact me out the blue after 5yrs. Also I hope that they do not think we owe this money when we have been paying it directly to CSA. P.s. My child is now 17 and have not saw him in 5yrs.