25f with my boyfriend 30m and I need advice on something happend recently

Hello, I (25f)am in relationship with my boyfriend (30m) since past 4 years. We met through a common friend and it was an instant connection. We started dating within 3 months of talking and he is the most amazing person ever. Now bear in mind this is my first relationship and his third. I didn't know what to expect but he was really supportive and patient throughout and I love him.

It is really common for us to go through each others phones like to order something, send something like pictures on WhatsApp, use insta etc. It has been 4 years almost and one morning I ask for his phone to check something and he immediately denied and got protective. I asked him why and he said there is something embarrassing in his phone.i insisted to see it because I thought it must be something funny but half an hour in me asking / arguing he still stayed adamant. But I did get to know he was using some kinda anonymous app and he said it's something related to his office and his friends were joking etc. I didn't believe his half assed stories obviously and said show it now or else I am gonna consider this cheating and he still didn't show that to me. I had plans and had to leave but I told him nothing will ever be the same if he doesn't show it to me and he still didn't. I left and cried in my car.

Later, he was apologetic and bought me gifts and all.

In my otherwise perfect relationship,this is the only incident but I just can't let it go. It has been weeks and I didn't ask about this to him anymore because I know he is smart and will come up with a perfect story now that he have had time to think. I have so many theories like did he text his ex? Was he making fun of me to his friends? Was he casually sexting/flirting?

Am I overthink this?

Tl:dr- boyfriend of longterm won't show me his phone. I think he is cheating.