Data Finishes quicker than before

Im very disappointed in the best network in the country. I have been using Vodacom since 2000 on another number. Then I changed numbers. I have been using the current number since 2006. I was excited when the data prices went down. Then I loaded 1GB last month, in 3days data finished. I then thought its me. This months I loaded another 1GB, 3days later data gone. I use to load 500mb and it would last me a months. My usage patine haven't changed. My data amount spent hasn't changed suddenly I'm going through more data in a week than before since this prices went down. Please tell me why my network is doing this to me?