6GB gone in 1 day

I signed a contract with telkom for a home router, 3GB during the day & 3Gb night data, on the 01st of June 2015 we received the data again but to our surprise the 02nd of June 2015 we are out of bundle, how is this possible when my wi-fi is protected with a password it is not open, how is it possible when we are at work during the day how can 2 cellphones us up 6GB in 1 day? Why are we having endless problems with all providers in in south africa, i am so tired of the service so tired