Get someone to take a vid of you riding. Chances are we'll be able to spot the problem.
+1, and pictures!
Oh god lol
Okay seriously though if you were on a 140 rental, there's no reason you can't ride a similar length other board. It may be stiff still as it's brand new, and may need a couple days to soften up a bit, but it's likely your technique.
Have you thought about taking a lesson on the new board?
Go back to basics and get yourself sliding again, it may just be taking you some time to get used to a new feel...