Why did this happen....?

Well, if you're not excersizing, that would be very helpful to actually lose the weight. If you want to get rid of belly fat, I recommend sit ups. I don't think mayo is a good idea for dinner because usually people don't burn the fat they eat at the end of the day, so if you want mayo, eat it with your lunch or breakfast, and have a light dinner. Also, a good idea would be to not eat anything after 8 pm. I hope this helps!