Business Ethics??

I have just run into a ethical dilemma...can you work for a company and then freelance for a rival company at the same time? Recently been receiving jobs from a different advertising agency and working as a freelance copywriter for them...I am still a bit sceptical as to the ethics that are involved...are you supposed to be working for a rival when you are in the employ of the company? Granted, i do all the work after office hours and in my free time...so no office equipment is actually used to help the rival, but what about the job done itself..what if, because of the work that i have done, the rival starts getting an edge above my company and thus cutting profits from it? A little voice in me is saying that i wouldn't have to do this work in the first place if my company actually paid me better...but there is still that nagging feeling that i am somehow contravening Section 25 of the Universal Declaration of Good Guyness... Sigh...