Change in monthly contract fee in the middle of my contract

Good day. Last year on black friday I got a data deal. The contract deal was 500GB anytime data and 500GB night owl data for R350 per month for 36 months. My total debit order was R358.99. This month I got the new invoice and by some or other way my debit order is now R417. I am utterly dissapointed in Vodacom. I signed up for R350pm x 36months. I didn't sign up for R350 for the first 6 months and R417 for the remaining 30 months. Its daylight *******.