Charged for upgrade and old contract

i did an upgrade which started on the 01st March , i get my statement and there is still R70 from my old contract that i have to pay every month till May. how can this be if i do an upgrade and nobody has notifid me about still paying R70 till May. if i had been told that i still need to pay R70 every month till May, i would have waited for my contract to end and then upgraded my contract. How can i pay R70 when i am not using the phone.