Illegal or legal? HELP?

I have worked for a retail company for 2+ years now. I have met my requirements and then some (including traveling out of state, overnight stays to help with store openings, and transactions such as returns and voids, which are both required to be done buy a manager) Let me also say I am only a sales ociate. I have been promised the next available promotion that became available for a year now by my DM, SM, and have already done more than was expected and what i'm actually paid to do. I became pregnant in August of 2010. Between the months of november and march it was said multiple times by multiple employees (including my DM and SM) that i would be promoted once returning from maternity leave. I returned from leave on May 19th (2 days after my 6 week check up) and was only able to work nights to avoid daycare. Out of the first 10 days I was available I was scheduled 8 of them, working 4 days (for 6-8 hours at a time) in a row. This was fine before being a first time parent but I didn't plan on working so much as soon as I returned. Regardless of my feeling I needed the money to make up for my leave. My first nights bad were HORRIBLE my daughter screamed the entire time and my husband called multiple times asking me to come home. The first night I just had to deal with it. But the second was unbearable. My manager is TERRIBLE with having enough staff. Which meant no one could cover my shift. Well after going through two of the worst nights of my life I texted my manager explaining to her that I needed the transition of me returning to work a little slower and asked if it was ok. She replied very rudely say "its a LITTLE too late" and many other things . I then asked if I could try and have a couple of shifts for the next week covered and she said "ok". So I did. The next day before going to work (no one could cover this shift) I missed a call from her but had a voice-mail from her telling me to work that night but that I had been taken off the schedule for the entire next week, (this is not what I asked for I just asking if I could have 2 days covered by a co worker). I tried calling her back and she didn't answer. So I went to work that night and enjoyed my extended time off. A couple of days later I received a call from a co worker informing me that my manager had filled the position I had been promised for a year now. In all honest I have worked my OFF, done my managers job, and ANYTHING they asked off me for crappy barely over minimum wage pay (remember i've worked there for 2 years) and the only reason i've stayed is for the promotion. What i'm trying to figured out is whether or not what she did was legal? I've read tin the FMLA and maternity leave act that you cannot be ped over for a promotion. Please any advice would be helpful.