Your probably smoking a heavey indica, which is my personal fave. Indica gives you a couch lock feeling, very heavey, and like you described, it's almost like an out of body experience. Indica dominant strains are the stronger, they have a higher resin production so a higher THC percentage. You should try some sativa strains if you want to be active still, and be able to stand without feeling the falling sensation.

Sativa can still get quite strong, but not as good as indica.

Indica are the really GOOD strains.