I need answers

On the 27 August I was surprised that a whole 1 Gig of data was gone as in less than an hour. I was shocked but thought probably I had opened a lot of apps. Yesterday morning I woke up and all my night data was gone all 3 Gigs and my night data was depleted. The message of the depletion was received at 1:15 am I was asleep round about that time.According to my understanding night data starts at 1am till 7am so I was baffled that 3Gigs would disappear in 15min. I decided to call the customer call centre and I spoke a very rude lady by the name Thembisile. She stated that according to the data she pulled I was using my data until midnight. Baffled I stated to her that I did not have data as it was depleted at around 7 so the only data I was using was my Whatsapp data but still I used that up until 9ish and I slept round about that time. Thembisile insisted that I used data up until 12am. She further stated I check what apps are turned on that run on my backround. I told her that social media apps are always off unless I wanna use them and the only apps I had using barely use much data honestly I rarely switch off my data at night because I'm very cautious on what apps I normally use. She further insisted that its my background data that used up my data. I proceeded to go with her theory and asked that if that was the case she meant to tell me that background data used 3 Gigs in 15 min? she refused to answer which angered me as it didn't make sense to me at all. This may not be a lot of data that I posed but now I'm terrified that the next time I recharge my data it will deplete before I use it. This is deeply upsetting that no solution was brought to me. Last night I purchased 500 MB of Data just to see if the same thing would not happen and nothing happened, I didn't even change the apps that normally run on my background. If someone from Telkom can please explain to me what happened to my data from the 27 and 28 because Thembisile failed to explain that to me.