Sounds to me like the centreboard Has swollen or a part of it.
When you sailed the boat did the board get stiffer as the day went on,if so it could that.
Is the boat wood or GRP, if wood the box sides could have swollen also as often when a boat is out of the water a long time the paint will come away from the wood and leave it bare,open to the water!
I would mark the centreboard so as to help find the bolt hole again,remove the board and have a lookat it,the box sides and also the keel each side of the slot for bare wood.
Could turn the boat on its side on grass and see the centreboard in action which may highlight the problem.
Good Luck CP 😕