Couple of possible red flags. Don't let them ruin tommorrow, but don't forget them either. We all have trust issues from what we went through. No way you want to go through that again.
But I'm not sure you should have asked her who was on the phone. It really was none of your business. And she knew that. And hence - the hesitation. At any rate - just go on your date and live in the moment. Stop worrying about everything. If she is not trustworthy - you'll probably figure that out soon enough.
