Is this illegal?

So I had a job that was a family owned business and I was told the turnover rate is bad. No one stays there and being there for a couple of months. I can see why. The system is pen pencil paper with everything and it’s super unorganized. Anyways the people that work there basically told me no one stays because of the system and the owner. But I stayed. Im pregnant and I’ve been sick as a dog so I’ve been calling in and I’ve offered a doctors note. But my boss wouldn’t take it. So I walked in for a meeting at 8am and they fired me for calling in. When I called in because of being sick to my stomach from being pregnant. I live in Oklahoma and it’s at will state so I’m not too sure. It might be legal but it’s shitty to bring someone in at 8am just to fire them.