Do you have a .rvmrc file in your user directory?
$ cat ~/.rvmrc
=> export rvm_path="/home/nick/.rvm"
It should look something like that?
And also check you've set up a .bashrc file like this:
echo '[[ -s "$HOME/.rvm/scripts/rvm" ]] && source "$HOME/.rvm/scripts/rvm"' >> ~/.bashrc