Installment loan by Check n go

I had an installment loan with Check n go in California. I think the amount was $2400. When I received the check the store front told me that I would have an installment agreement of $289 every 2 weeks! I think the total installment total was 16. They gave me a list of installment dates, and I told them, I cannot pay except on paydays 1 & 15th. The guy who did the loan said that ok, we will just hold over the installment till payday, which was fine, but very confusing as to when the last payment would be. I kept asking towards end of Feb. when is my last installment they told me it would be End of March. When I went in end of March they asked for 2 more payments. Keep in mind that mean 2 installments of $289 are interest only. I told them, I am not giving you any more money. YOu have told me for months that I only owed till end of March and have not given me a correct installment agreement with dates of my paydays to date.
So, they tried to cash the checks, I stopped pmt on them. Keep in mind I have paid over $4,000 in pmts, I have more than paid the principal. I now am receiving calls from collection agency at work. This was not a payday loan, it was installment loan, they called me and solicited on phone.
Do I have a legal right to withhold last 2 pmts?
Signed.
Sick of Bottom feeders!