On a 20 Gig data package but after 5gigs have been used I can hardly do anything on my phone.

I have a 20 gig data package with Telkom number ********** 868 - My total Telkom account is over R3000 per month. After doing a balance enquiry and having 15 gigs of data remaining my phone is as slow as a snail. I have called the call centre 4 times to log a complaint and all I get told is that "My Free Me data is depleted so I'm now running on a slower speed. I mean that is not acceptable at all. Why have me pay R749 for a 20 Gig package and then rip me off like this????? I feel that it's a total ******* if I can hardly use my phone... I mean don't advertise and say that I will be on a 20 Gig data package and then just like a week before end of the month I get the sms stating that my Free Me data is depleted... so because my free me is finished I now have to suffer.