So I fell of my bike.... Do I need a new helmet

[CaNsA]

There's no way of telling if the inner material has compressed by the status of the plastic shell... Thus buying a new one is the safest thing to do.

How is the inner shell going to compress when there is no direct force applied?

 

[FunkyCowie]

You should replace the helmet after an impact, even if the internals/externals look fine the plastic shell can have micro fractures that will cause, on next impact, it to crack open like an egg making the internals rather pointless.

 

[bloodiedathame]

How is the inner shell going to compress when there is no direct force applied?

Seriously? The outer plastic shell will bend in/deform on impact (ie a drop) and compress the innards.