Data overages are bleeding my wallet dry

In total, my bill is now $300 or more thanks to $20 overage charges (per additional 500 MB). Thing is, several of these overages occurred when I was on a WiFi network. 10 minutes ago I received a message saying that more than 75% of my additional data had been used. I'm at home, connected to my WiFi, and yet I receive such a message! So what can I do but disable cellular data on my phone? Data disabled, I go back about my business when not two minutes later I receive a message saying that 100% of my additional data had been used! What the heck?

I'm just so sick of this phone bleeding my wallet dry. But I'm stuck with it for at least another year because I can't afford to get out of my contract. I wish ATT could at least remove the last $20 from my bill, because I sure as heck did not use 75%, then 100% of my data when I was connected to WiFi at the time and had used very little cellular data the night before. But I somehow doubt they'd do that, so I all can do is vent my frustration here.