Was you using the same ammo?
Check to make sure the mount or rings have not become loose, it does not take much to throw a group off. If they are loose try using loctite next time.(Sometimes you can shake the scope to check looseness other times you can not physically tell although it may still be loose)
If you have disassembled and cleaned you rifle recently also check your stock screw(s) and make sure it/they are not overtightened or are tight enough...I cleaned my rifle one time and overtightened the stock screw and it totally through off my zero...as I said before , it does not take much to throw off your groups.
As for the warranty the company will(should) check for errors when you send it in and repair/replace accordingly(if the scope is bad its bad, you should not have to "prove" anything), but I would try all other options first.
