Airtime missing

Loaded Airtime on may phone of R30 and bought a 100MB of data. I waited for about 5 minutes. I then Switched my data on, so bbm messages came through. When I checked my balance again I had no airtime and no data. I phoned the customer care-line to ask what happened they said I didn't buy data bundles and my airtime was used up by the internet. I don't understand how R30 airtime can be used up by about 5 bbm messages.