I googled "smoking the carpet" and found lots of quotes and references. It means that one is high on cannabis (possibly other drugs?). I have not heard this in BE, however. Having said that, I don't smoke dope, so it is possible that it is current in BE.
I've heard the cat thing too, but that's a specific situation. Piotr190 is refering to an expression. Probably because the rug can be rolled up, resembling a joint. In this case, it would just be a reference to a really big joint (like in History of the World, Part 1), meaning that the person is really high, or acting like it.
Hemp is another name for the marijuana plant. Aside from the chemicals (drugs) that can be extracted from the marijuana plant, the fibres can be (and often are) used to make rope, coarse fabric, etc. Accordingly, it is often woven into rugs/carpets and the term "smoking the rug/carpet" is an idiom used to mean smoking marijuana.