I felt sick, very lightheaded, at work 2 weeks. When I called in sick the following day, I was told that I lied, that I was unprofessional(have even a letter from them) and that I am not allowed back at work! I also had a doctors notice but didn't help... this past Friday my headaches were severe and I ended up at the ER and they performed several exam and scans on my head. The doctors told me that there is a certain mass on my brain and need to go for an MRI this week. The company I worked for for 15 months is a trading company in Chicago, trading fixed income products at the Board Of Trade and had no official contract with them! Do I have a case here to take these people to court?
Thank you

Most US States are at-will states, which means, unless you have an employment contract or are member of a union, you can be fired for any reason that doesn't involve being a member of a discriminated class.

However, if they have a published sick leave policy and they ignored it, you may have some cause of action. Consult an attorney to be sure.