Hi there,
Could be off-collimation, but if the scope is well-collimated, most probably it was a tube current. It happens to my views when I don't ventilate the tube, even when it is cooled down.
Do you have a fan on the rear of the tube?
Try the star test: if on one side of defocus the star image has a spike and on the other it is somehow flattened, that's the tube current. If the stars in and out of focus have spikes, than collimation is needed.
You could also rotate the eyepiece and see what happens to the spike to eliminate eyepiece errors.
Finally, rotate your head. If the spike rotates too, that's your astigmatism.