Back wheel suddenly started "wiggling" I bought recently a second hand bike for my daily rides, and it seemed in good shape. I rode it for more than a month without any problems, but recently had issues with it and had to get it fixed (mostly about a twisted rear wheel)

Out from the shop everything was fine until this morning, when I heard a "clack" coming from the back wheel, and suddenly felt friction on it
Upon inspection, it seems like the wheel is "wiggling" by like 1cm. Both screws on the side are well tightened and do not move.
The axle is not moving in the rear dropouts.

What could cause a problem like this ?

(I'm fairly new to this, so if it is a common issue please let me know as well)

EDIT: The problem seems to be a broken axle, I'll close after checking the bike again.

EDIT 2 (SOLUTION) : Upon inspection (sorry, couldn't find the time earlier), it's definitely the axle. I looked carefully, and the ball-bearing seems busted, with some balls out of it. I guess I found the problem, thanks for all the answers
This could be due to the cones on the axle coming loose. The cones screw onto the axle and prevent play in the bearings. If they are too loose the hub will be able to move independently of the axle. If they are too tight the wheel may not turn freely and you may damage the bearings by crushing them. See here for more information on how to properly adjust the cones.