Hello.
Best thing you can do is not panic. Stressing yourself out will get you nowhere.
If you have not plagiarised, then you have nothing to worry about; do not let anyone accuse you of something you did not do, you are a legal adult and you can if they're your own notes, prove your competentcy verbally over the subject material.
Another thing to remember for the future, even if the exam is open book try to do some of it without using your notes.
I know it sounds stupid, but open book exams can be worse than closed book ones, as it's near impossible to plagiairize in a monitored closed book exam. It's however very easy for students to plagiarise in open book exams. So any "high percentage" to your marker will automatically be questioned.
Regarding the notes you used from the lectures... The notes that your lecturers use most of the time are works from online and scholary academic sources. These sources are published and most of them are accesible to softwares like Turnitin and SafeAssign's databases; so if you've used lecture notes, and they're similar to what's already been written, the overall similarity score will be high.
If you do most of your work from the top of your head and try to credit ideas you use (e.g. Smith argues this....), then most likely your similarity scores will be lower. You'll also be citing where you got the notes from; so no one can accuse you of 'plagiairsm'.
So try not to use your notes in the future. Revise well and if you need to, refer back to them with a quick glance for open book exams.
As said before don't panic, similarities are inevitable sometimes.
I remember my frist University submission came back in the middle end of the green scale to "past student papers" (around 10-11%), and I'd used my own ideas and cited all the other stuff I'd paraphrased. I couldn't help that. It was a coursework paper on a subject that had been done to death, so some of the stuff I'd mentioned had been written probably thousands of times before by other students. Nothing ever came of it though.
Try not to worry, and prepare your argument well.
You are innocent until proven guilty! Don't let anyone tell you otherwise!
Please let us know how it goes.