Hospital rip-off

Good day

Was in hospital last year October for a scan using my medical aid. 2 weeks ago I received a call from Meranda that the scan was not paid for by the medical aid, fair enough invoice should be sent to me. My question is why did it take them 7 months to contact me and I was told that they did not have my numbers how I don’t know, cause on the day I filled in an application form. The amount is R883 and I’m being told if I won’t pay by the end of June the amount Will generate interest. Which I failed to understand cause they kept the invoice for 7 month’s. Now the new bill is R1400 I just wonder how did they calculate the interest and who agreed to be billed with interest? I urgently need proper explanation on who’s responsible and why must I pay the bill immediately?