what do I do?

I need some real deal advice. I've been with the same man for 5 years. I love him with all my heart. My family loves him and vise versa. Things wouldn't be the same without him. The first month we started dating I guess he messaged a girl on kik and sent her pictures and received some back.. he claimed that he typed in a random group and it came up. I never had a kik so I'm not sure if that's true. I too a couple years later cheated on him. We never broke up just forgave each other and we've been better then ever. I think we realized how much we need each other and that other people aren't for us. ANYWAY! For like a year now I've been having this gut feeling that my best friend has feelings for him .. because every time I'm with him she'll be like "where's so&so" "he looks so good" what time does he get out of work.. all questions .. if I vent to her about him like oh I think he's being fishy or if we got into a fight and I'll explain to her what happened she always sticks up for him .. it bothers me. She'll text me and ask for him out of the blue like wondering if he's home. And sometimes he'll notice and say something about it. I feel like something happened between them and neither one of them want to me to know. I asked my boyfriend multiple times. And he gets so upset that is acuse him of something like that because he's like I almost lost you once don't want to do it again. I told him I want to take him to a lie dector place and he gets so pissed off I mentioned it yesterday and he got angry. And told me I'll probably fail.. Idk what to do