It sounds to me as though you signed a contract entitling the lender to change the number and amount of your installments . you should have read your terms and conditons . Its really bad but the contract can be held up in a court of law , Im sure they never told you you would have that nice surprise at the end of your loan , but you didnt ask. i guess you know now .
you would be best to argue a reduced settlement or spread out the payment .
but if you arent satisfied with that I would take it further and see what an ombudsman would say .