Discriminated against and fired at work for pregnancy.?

So, I told my boss about a week and a half ago that I am about 5 1/2 months pregnant. Then the other day I told him that I've been feeling kinda lightheaded lately and might have to sit down and take a breather for a few seconds on occasion. Then 2 days later, today, over the phone he tells me I'm not a fit for the company and fires me... even though I make the most sales (we are a retail store). Personally I think he thinks I'm a bad image for the company's sales and crap because I'm round and pregnant and I'm going to need to sit, etc. as my pregnancy goes on. Is this discrimination? Can I start a case or what? How do I do that?