Is work in the wrong or am I?

I had called into work with nausea, vomiting and other things. They asked me to get a doctor's note for it after asking how far along I was. They then remarked that it was "too early for any kind of morning sickness to show up". Then they said they'd fire me if this became a monthly thing when I have only done this once and I've been working there for only 3 weeks. Who is in the wrong? And what can I do about it?