Where were you?

My boyfriend told me he's going to train more. (He does karate.)
He has official training on Tuesday, Thursday and Saturday.
And he wants more?
He's not doing this alone. There are 2 who are going to train with him.
One boy, one girl. This is were I have a problem.
What if the boy gets sick or can't come?
My boyfriend alone with a hot girl with a cute accent, doing a lot of touchy "karate".
I know I should trust him. I do. But it just makes me uncomfortable.
If someone would ask me: "Are you a jealous person?"
I would have to admit: "yes, I am."
And so is he. So we're a perfect match.
When your boyfriend says to you: "Honey, I'm going to train some more. Don't wait up." or a call you get: "Don't wait up, I'm going to work overtime."
Do you hear those warning-bells?
A warning inside you, that tells you: "Watch out. Is he really doing what he says he's doing?"
I've seen too many situations were a guy says he's "working overtime" or something like that and he's actually cheating on you.

I'm just being paranoid. I'm just going to have to trust him. Right?