DVI-D supports 144 Hz, HDMi 2.0 supports it too, so does Displayport. I would guess you're using a normal DVi cable. But then, if it ran at 144 Hz before then that surely is wrong.
DVi-D cables and Displayport cables are pretty cheap so getting another may be your best bet, should solve the issue as then you will know for sure that all of your hardware supports 144 Hz.
Did you go into Nvidia control panel to change it to 144 Hz?