Do you have a fireplace or wood burning stove? Mosquitos hide dormant in dead wood over the winter, so it's not uncommon to bring in some logs from outside and have the mosquitos inside wake up, thinking it's spring in your nice warm house.

I learned this when I was getting bites inside my own house with sub-freezing temperatures outside.

Same and same. I kept wondering when they would finally die off. They were big fuckers too!