Microwave-safe cups becoming less safe
Whenever something gets hot in a microwave, it indicates the presence of water. If something that used to stay cool suddenly heats up, you have water present where previously there was none.

I assume you are using a glazed ceramic mug or jar, not one made of glass. Your glazing must have tiny cracks in it, that allow water to reach the clay underneath and collect there. Basically the wear and tear of long use. These cracks may be so tiny that, especially if no discoloration is present, they are virtually invisible. Each time you use the cup in the microwave again, the expanding water will actually aggravate the problem. The same is true for running the cup in a dishwasher, which in my experience allows for more water to seep through the cracks than a quick hand wash.

Time to get a new mug. Consider one made of glass or porcelain (china) this time.