My data keeps disappearing

Today I’ve purchased 150mb 3 times and my data keeps vanishing, I use an iPhone 7 ive been using Apple for more than 4years now all my applications are switched off but WhatsApp, usually 100 mb lasts me the whole day but not today 150x4 = 600 doesn’t last me 2hours what’s going on I’m writing my final exam and me and other students are sharing information via WhatsApp as we usually do but I’m delayed and behind with my studies! This is not only costing me money wise but my career is on the line I can’t even go to safari to do research in fear of my data running out. I’m not pleased and I’m getting frustrated. Please don’t tell me about 4g and other apps running. I’ve been using 4g and Apple for a long time I know how to be cautious with data this is a *******