I'm starting to feel uneasy about my boyfriend's new coworker.

I've been in a relationship with my boyfriend for just over five years now. Recently, he received a promotion, and they brought in a much younger woman to fill his previous role. Ever since, I've been sensing some unusual vibes. To start, after her interview, she privately messaged him to thank him personally, even though there were two other managers involved in the hiring process. She invited him to reach out if he needed anything and expressed excitement about hearing back from them. He replied, complimenting her interview skills. I want to clarify that I wasn't snooping; I was sitting right next to him when he responded. I find it a bit unprofessional for a candidate to reach out like that instead of sending a standard thank-you email, and when I mentioned it, he became defensive. They've been exchanging texts throughout the week, which is understandable since they'll be coworkers moving forward. He hasn't hidden much of what she's said, but some of their conversations seem a bit flirty. I also noticed him looking her up on Instagram, which he rarely does. Given her age and attractiveness, many of her photos are revealing or feature her posing in the gym, which makes me uneasy. With her training starting soon, they'll be spending a lot of time together, and it's increasing my anxiety. For context, I've never felt this way about any other girl or situation during our relationship—this is a first for me. I'm not quite sure how to approach this. He isn't outright hiding anything, but after a couple of weeks of working together, she's begun calling him "papa," which strikes me as odd. I'm unsure how to express my discomfort without coming off as jealous.