<human>: Summarize the given text in a few sentences.

This morning, I woke up a little earlier than usual and went on my ipad to waste time until I had to get up and ready for work. I was mainly dicking around on social media, checking facebook, twitter, etc. when I opened Tumblr. 

I don't use Tumblr often, so when i saw that i had a message I checked it, only to discover that my BF had logged in and the message was sent to his account. It was from a girl I don't know, asking why it had been so long since they had talked and that she missed him. Since I didn't know her, i kind of went through the messages to see if she could be a mutual friend from somewhere or something, and that is when i discovered something not so great.

He had multiple pictures of her naked, and there was some graphic writing between them, mainly about S&M and some kind of Daddy/little kinks. I was shocked as he has NEVER shown any of that around me. He was also lying to her about his profession, saying he was a teacher (he isn't) and that he's going to grad school (he doesn't even have a bachelor's yet).

I don't know how old these messages are, since Tumblr doesn't time stamp any of them. I don't know if they were before we started our relationship, but he mentioned being 20 in them and we started to date shortly after his 20th birthday.

I don't know how to approach him about this without seeming like i intentionally snooped. I really did think i was on my Tumblr and i normally would never go through personal messages without good reason or permission from him.
<bot>: My boyfriend left Tumblr logged in on my iPad and i found unsavory messages between him and an unknown girl, including nudes and talks of fetishes I had no idea about.