Fed up on data confusion

Im actually so fed up. I purchase a R30 hour 1 GB data bundle so I can help train my clients online. But Ive noticed the last 2 times that MTN. Takes up all of my current data, the last time we didnt even get to 48mins on whatsapp video call, it took ALL OF MY DATA. Which it shouldnt, it should use the 1hr data bundle and THEN tap into the phones current data. Ive NEVER HAD THIS PROBLEM for 6 weeks now all of a sudden MTN just takes and takes.