A couple of things could be the issue. Its been stated before and I agree, hamstrings. Not even a weakness issue but more of an activation issue from being quad dominant in squtting up until you switched your stance. Make them a priority in training if you want to continue using a wider stance.
The biggest issues are going to be:
-Weak hip external rotators/abductors
-Tight/shortened internal rotators
Close stance squatting doesnt require the amount of external rotation (i.e. when people say push your knees out or spread the floor with your feet) as a wider stance squat. Start doing every rep with your knees driving out has hard as possible. This will be very difficult to squat this way at first because your external rotators are probably very under developed.
The lack of external rotation puts more stress on your internal rotators, which should act more as a stablizer than a mobilizer. All the muscles that make up the medial part of the quad, adductor complex, and hamstrings become all bound up in one another. This is called a sliding surface problem. Restoring sliding surfaces is pretty easy but hurts like shit. This is where all of the foam rolling, lax ball, mobility, and any other soft tissue work you can get your hands on would be extremely beneficial.
So, my suggestion:
-Get your hamstrings strong as shit
-Focus on external rotation when you squat- Knees out
-Do hip mobility, with an emphasis on your internal rotators, every day until squatting feels comfortable with your new technique