Now im employed, what happens to my personal development?

Hi all,

I recently got a job as an Actionscript Developer but before i did that i was making very good ground with Box2d physics engine.

After a couple of weeks, im looking to get back into my own experiments, but what happens with regards to my job?

Lets say i create a roller coaster game over the weekend and put it on my website as an experiment, can my employer then say “i want that game” and ill loose the intelectual rights to the project?

Im not really very sure about this stuff so it would be cool to know. Also, when i do work for this company, is it possible to put up screenshots on my portfolio or again is that not allowed?