labrador unable to support herself with back legs

Question
my 13 year old labrador within 1/2 an hour went from walking ok to not being able to support her back legs we have laminate floors is it possible she may have strained a muscle slipping on the floor is there any over the counter pain killer i can give her to relieve any pain she may be in

Answer
The only safe OTC pain killer is aspirin, preferably buffered aspirin.  A regular 325 mg tables every 8 hours should be about right for most labs.

The slick floors can be treacherous.  We have a hardwood floor with an industrial grade urethane on it.  Young visiting Labs do a lot of sliding around.