Why are pregnant women get like this?

There so miserable and difficult to deal with. I had a pregnant teacher in middle school who was super kind and happy but when she was about 4 months she started to not act like herself and stopped smiling like she normally did. I felt really sad because she was a nice,happy,bubbly, teacher that always smiled but after a while she became this rude and angry teacher. Some mornings she never even say “hello”! She would get angry over 1 little tiny piece of paper on the floor or start yelling when calendar falls off when no one even touched it. One afternoon when it was her 7th month of being pregnant and a group of boys in our class were playing a game that involves pushing and bumping into each other during lunch time and one student got hurt and had his head on the desk and when she came in and saw it she started to get super angry at how people are getting injured and pushed and screamed at us for like 10 minutes! She didn’t even calm down once !! She just kept yelling and yelling and said “ IM SEVEN MONTHS PREGNANT!! AND THIS IS THE STRESS THAT YOU GUYS PUT ME THROUGH??!!” I was legit terrified of my teacher after a while because everyday she gets angry over the LITTLEST THINGS and never put a smile on her face. I even gave her mini happy notes telling her to stay happy and positive but she still wouldn’t smile or act happy! She liked them though. So why do pregnant women get like this? And get getting mad at the littlest things we did?