Fishy? Or am I paranoid?

Ok so I’m 17 and my bf is 18. In the middle of us having sex he gets a call from a girl I don’t know the name of. He looks at his phone and said “ I don’t wanna deal w this shit” then after the sex the girl that called him texts him ( Ik this cz I was siting right next to them as they were texting) anyways... she was like “pick up ur phone broo” like tf. And he said he couldn’t cause he was “at work.” Mean while he was w me. So I was like y would u lie and say ur at work. And he said I just dont feel like talking on the phone. Then she said “do u want me to pick u up on Friday morning.” I asked him what this was about and he said this is his best friends older sister (she’s 22 he said) and she works at a dentist place for school and cause she’s starting out she needs like 50 patients so he said he’d get his teeth cleaned by her every so often. This was the first I heard about this and even tho I trust my bf it just feels weird to me. I even asked him y she would have to give him a ride and he said it’s cause last time he couldn’t find the clinic. I didn’t even know he’s went there. After tho he asked her what time he’d be done and she started talking about the clinic so I guess it’s true. We’ve been together a year and a half. what do u guys think?

Edit: should I wait and talk to him abt this in person (problem tho idk when I’m seeing him next cause he’s sick) or should I just text/ call him abt it cause it’s driving me crazy

Edit// I talked to him abt it. I honestly believe him and I don’t think he’d ever cheat. He said he wishes he never said yes to it cause he doesn’t want to do it. The girl that called him is his best friends older sister so they’ve known each other for years.