Those are usual suspects...Are you weighing your ingreadents for your sauce or just eyeballing? Being unbalanced with your measurements could be the issue.
Have you changed any of the ingreadents,ie milk fir full fat to 2%? Typt of fat or flour in your roux?
Just some food for thought.
if you changed cheese and went for a more aged and harder cheese, these tend to break up and release their fat more easily than soft cheeses.