Can I be let go for this?

I've been in and out of the hospital with this pregnancy. The past two days I've had a severe stomach flu that was causing contractions from being dehydrated so I was hospitalized. Before this pregnancy, it was rare that I called in and I worked a lot. I've been with the company for 2 years and have sick pay saved up. They were angry with me yesterday because it's difficult to find someone last minute to open the store but I was in the hospital and there wasn't anything I could do. Can I be fired for missing so much work due to medical issues? I have doctors notes. My job keeps mentioning me filing for LOA but I wanted to save it for my maternity leave. I'm stressed out and anxious. 

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