Maybe it's not even a sudden change of mood. Maybe it's just been a downward spiral during the day. Maybe one day they've been happy and the next day they're not.
In my case, it's been a slow spiral down during the day. I felt alright this morning, even though I had to get up early for a lecture. But I had a break between my two classes today and during the break, I thought 'I should do some reading for a class'. I started reading and got through maybe a page and a half or so, but then I started feeling really sleepy, seeing as I didn't get as much sleep as I should have last night. So I decided to lay down and rest for a little while.
Then this afternoon, I decided I should do some reading for a different class that I have tomorrow and answer some questions that I need to answer. But as I'm sitting here, reading and answering questions, all I can think is 'ughhh, this is so boring'. Also, a little bit of 'these questions really aren't that easy to answer'. Then it gets me thinking about what it'll be like to write up proper lesson plans for when I go on prac and how scary that will be, having to teach a class without any assistance. And I'll be getting marked on it and I feel like there is so much expectation to do well with my assignments. But I can't do well unless I do a lot of the readings, but there's so much to read and so much information to take in and ughhh, it's kind of doing my head in.
Then it gets to the point where I over-think so much and my brain thinks 'maybe you're not meant to be a teacher. Are you sure this is what you actually want to do?'.
Shut up brain. I don't want to hear you.