Disclosure: IANAL.
Adding to John's message above:
>> Go back to the employment agreement you signed, read it.
Not just read it but make sure to read it very carefully. There are terms frequently used in such contracts that look like normal English (or other language) words but may have very well-defined and specific legal meaning. Examples would include "work", "inventions", "work for hire", etc. Generally you can find definitions of these online, including in Wikipedia, and also sometimes in the contract itself.
The contracts also often have a place for "excepted works / inventions" where you can write things that you would want to exclude from the contract. Be careful again of the precise contract though as:
1. The exceptions are generally intended for work you did "prior" to employment by the company. Anything you do after joining may not be covered in this form.
2. As John noted, you may still have a requirement to disclose any excepted work you do within prescribed time limits to the employer.
3. You may have non-compete clauses which prevent you from working on things related to the employers line (or anticipated line) of business. This bears another example where people often misinterpret -- They claim for example that their side project is on end-consumer software application while the company products are enterprise related [Hint: Do not go by your or normal language definition of "related". As far as legal world is concerned, "software" may be just one category. A good check, though probably not correct, would be to see which line of businesses the employer's trademarks are registered for.]
To summarize:
1. As John said, better to discuss with a lawyer.
2. Even with '1' above, go read a book or two. E.g.: "www.amazon.com/Intellectual-Property-Open-Source-Protecting/dp/0596517963/"
3. Do not blindly fill the excepted works/inventions form. It by itself may not save you. You may need a separate agreement in writing stating something like "Employer has no interest in your side project and does not mind you working on it on the sides". Make sure also that your side project is well defined in this new contract -- My guess is that over including or under including would not be good ideas.
4. There have been previous discussions on HN. I would have found some for you but need to rush right now.
Disclosure: IANAL.