It actually took a bit of time to sink in - I mean, I have a pricey plan, but, uhhhhhh...
It turns out that they switched over my unlimited data plan to one with unlimited MMS, but only 5Mb of data. They claim I went into a store and changed it in May, which is interesting - the only thing I did in a store in the last few months was get a new sim card, and last month's bill still had unlimited data.
After about 45 minutes on the phone, they claim they'll fix it - at least, they've filled out the paperwork to fix it. I should check my bill in 5 days, and see if it's all better.
On the plus side, the woman helping me, while not the brightest in the world, was actually concerned about the situation, and as far as I could tell, did her best.