How do I write a thriller story in which a character needs to decide how he feels about a recurring person in his life?

Hey y’all! I’m writing a story in which the main character runs into someone that he knew as a teenager a few times in the course of the plot. The first time he sees this person is incidentally, but from the second time and beyond, he starts to wonder if this isn’t just coincidence, but if his old friend might be stalking him. I want to keep his motivations kind of fluid for a while, as he isn’t sure if he’s bothered or grateful to keep running into her while he’s dealing with the mystery part of the plot. I also don’t want to give away too much of her motivations very quickly. I thought about her backstory and I don’t want to show it until the very middle of the book. <br><br>How do I convey all of this to my readers without making them confused or angry? Should I write an alternative POV for her at some point? I have so far kept all of the POV to the main character.