Using heavier strings may have affected the nut slot. Check that the lighter strings are not sitting too deep in the slot. If they are, the best fix is to cut and slot a new nut. (A good excuse for upgrading the nut to bone or similar). An ok fix is to make some shavings/dust from an old nut, mix it with superglue and fill in the slots. Let it dry, and then cut new slots to the required depth. If the neck shape has changed, then a sight along it should show you the amount of relief in the neck. If it is straight, or bending slightly back away from the fretboard, this may be your problem. A quick truss rod adjustment usually does the trick. Without seeing the guitar, my best guess would be the nut.
Tim from Bit on the Side