My question involves labor and employment law for the state of: Iowa

I am 34 weeks pregnant. I am an assistant teacher, working with 3, 4, and 5 year olds for an Early Childhood Center (daycare) inside of an Elementary school. I am employed by the schoold district. I received an email from one of my superiors stating that I am responsible for pushing a 50 lb + lunch and breakfast cart outside in the snow and ice on uneven ground and up a ramp to a 'portable' (old mobile home turned into 2 clasrooms). I expressed my concerns with being pregnant, and the risks involved with doing said duty. I got a doctors note limiting me to not push, pull, or lift anything more than 25 lbs. I turned my note in at 8:30 am and I received a call from my boss at around 1 pm telling me that I have to start my maternity leave (6 weeks unpaid) immediately because they cannot accommodate my medical needs and I am a liability because I'm unable to lift more than said amount. It is a liability for them to have me push the cart on ice and snow because of the risk of me falling and being injured, yet I was still told to do that job. Also, I was allowed to work the rest of the day with instructions to "just be extra careful". If I'm such a liability how was I allowed to even finish my work day? Is this legal for them to do? I have 2 children, with my 3rd on the way and me being off of work without pay for 6 weeks will drastically hurt us. I don't know what to do, and am incredibly worried for my family's future.