If the drop knocked the optics in your lens out of proper alignment, it will be most noticeable at the wider apertures. Narrower apertures that give greater depth of field can help mask misaligned lens elements.
You need to have the lens checked out and probably adjusted.
Based on personal experience: If you send it in, be sure to include some sample images that demonstrate the problem and also a detailed written description. Use images that show a clear focus target and also areas in front of and behind that target. I like shooting at a hash mark on a lined U.S. football field. By shooting at a low angle, the detail in the grass around your target will show what areas are and are not in sharpest focus. Another good target is a sign posted to a chain link fence. Shoot one shot straight on perpendicular to the sign and another at about a 45° angle to the fence.
I've gotten lenses back that are much more correctly aligned when including such images than when only giving a written description.