What causes this, and how do I fix it?

[pji]

These are 1mm vertical walls. I've been printing these for months without any issues, but a week or so ago, this started happening (see image).

I swapped out filaments .... same results with each.
I changed the nozzle .... same results.
I leveled my bed.

I can only imagine that it's an extruder mechanism problem, or maybe something in my slicing software.

Can anyone help me? I'm a relative newbie (4+ months).