Workplace trying to scam me

Hello!

I am a seasonal worker in Norway. Ive been working here for 2 months now.

I signed my contract 4 months ago and everything seemed okay. I have agreed to this contract because of the good terms, it says after 9 continous hours I get 40% overtime payment on the hourly pay.

I noticed that on my 2 payslips there is no sign of overtime payment eventhough a lot of 10-11 hour days have been done.

Now the company is forcing me to sign a new contract in the middle of the season that says I dont get overtime at all. The reason being, they have a plan to save more money.

This doesn't seem legal. What can I do?