This is a challenging situation for anyone. The seed of suspicion is now sown and from what you describe, rightfully so.
Given the length of time you've been in your relationship, the reaction of you turning up sounds a little strange, to say the least. Further, I also presume you two should have a certain amount of trust and transparency too. Bear in mind, women are probably far better at concealing adulterous behaviour as they're better equipped at planning long term.
I also asked my wife, Pam, for her thoughts from a female side too. Her reply “There's something going on if she won't even let him go and use the toilet. That's very odd”.
While I'm sure you're upset and probably angry at what's likely to be going on, you also have to approach this with a clear head and be level headed too. It ‘could' be a comedy of errors.
While I know you've already mentioned to her about the actions you took by waiting in the car for over an hour and she deflected the situation, there's got to be some next steps. Be prepared that she has cheated and what you're likely to want to do in the relationship next. Be prepared that there was another reason for her actions – she might have just wanted to be left alone, for all we know.
At some point, you need to quietly sit down with her and express your thoughts and concerns. It's OK on some level if she wants someone else. It's OK if she deceived you for whatever reason. It's OK that you need to quit the relationship, due to her actions. Trust must be a core part of any continuing relationship and you both need to be honest and accountable for your actions.
It's a matter of finding the truth.