Arrears- Can CSA go back further than last assessment?

I have been involved with CSA for about 11 years, I was initaily assessed to pay £15.40 aweek due to being on a low income and paying high rent. I have allways provided finantialy  for my children on top of that since things improved for me and thats how it’s been util recently my ex asked for me to be re-assessed and I now pay £72.00 a week plus £20.00 a week arrears.  My ex is now saying I should have been paying that since the begining and is going to ask the CSA to recover that money from me, even though I have been having my children 3 nights a week, buying all they need, paying school dinners taking them on hollidays etc. etc.

Can the CSA go back past my last assessment?