Sorry, I cannot tell you why you're using more than you did before, but I feel that in this day & age, 3Gb is a very low BB allowance. That is unless you're a very light user.
If you don't mind me asking, which ISP do you use & how much do they charge you for the 3Gb? What is their minimum contract, & how far are you into it? Have you considered changing your ISP to one that gave you more for a similar price? G