20GB that lasts me for a month only lasted 2 days

I got a monthly data allowance of 20GB. On Sunday, May 5, 2024, at 11:56, I had 19.94GB, then to my surprise, at 18:42, I received a notification stating that I was left with 7.90GB. I'm concerned since, between both times, I only spent a few minutes on my phone because i was travelling. On May 7, 2024, around 11:00 a.m., I received a notice stating that I had run out of data. I don't understand how it was reduced; 20GB lasted me two days, which is impossible. I contacted Vodacom, but I am not receiving any assistance. 20GB used to last me a month, but this time it was just a few days. I am not satisfied with Vodacom.