The phrase for today is another great one that you can quite easily.
" Have something on my mind "
This means to keep thinking or worrying about something.
Example 1
A. Hey Sarah... Sarah... Sarah!
B. I'm sorry. I had something on my mind.
A. What?
B. My work.
Example 2
I couldn't focus on my TOEIC test. I had something on my mind.
https://idioms.thefreedictionary.com/have+something+on+your+mind