You said it worked fine before the tension dropped.
What was the original tension? Maybe it was too low.
Probably a problem with confidence more than anything else. Calm your mind, go out and hit again.
The stiffness and the weight is too similar for such a huge problem. And if you're slicing instead of topspinning strokes, it's a mental problem.
But tension is one thing you left out, especially since you think it dropped after one hour of hitting.