Clutch pedal play was 1", now it's 2 1/2". What happened?

[Neil Mylar LakewoodCA]

I installed a rebuilt transmission in my truck, 2 years ago. At the time I adjusted the clutch pedal play to be about 1 inch.
I've only driven the truck 4 or 5 times, since then. The last time I drove the truck, the transmission kept grinding. I recently check the clutch pedal play and it's 2 and 1/2 inch.
What could have caused this sudden change? I am going to attempt adjusting it tomorrow.

Thanks in advance.
Neil