CSA are making up figures and saying I haven’t paid when I can prove it

I have had a weekly standing order with the CSA since October 2000 to present and still being paid my son is now 25 I have just got a letter from them saying I owe £8000 I have rejected it which they have come back and said it is now £5000 didn’t explain why it had changed but when I asked how have I got arrears as I’ve kept standing order going they said case was reopened in 2006 and payments increased which I was not aware of this as have had no contact of them also looked into it and my son started work in 2006 but they say it’s up to me to tell them although I’ve had no contact since he was 2 also checked payment and I’m over 300 payments missing again told to prove it. I’ve got proof the last ten years have been payed every week now in progress of getting years before that I would think everything they have or as you may say have not done over the years that I now have to try and prove which is hard to do in some cases ie his leaving date from school I’m getting more and more stressed and wonder if this nightmare will ever end I don’t know we’re to turn anymore.