How many red flags does it take?

So my girlfriend and I have been dating a year and a half now.

To get right to it, she told me that she has a friend of his that is like a brother, who I know exists but have never met. He is in the Army and come to find out they have been sexually active for a long time and that his wife and he have an open relationship but to my knowledge nothing since we have been dating! She has been sending messages on FB and through SnapChat!

She also plays Words with Friends and I saw where he has been talking to a bunch of different guys, one in particular, where the conversation went way out of bounds. When asked about it she said that she was trying to make him feel good about himself as he was on many dating websites with no luck.

She contacted a guy who he was talking with on a dating web site just to say hi and again when asked about it she stated she didn’t remember doing it.

She has a co worker who she texts and IMs a lot, to long to get into, but definitely inappropriate messages in my opinion, she says that he has been over to her old apartment but nothing ever happened. It was just watching movies and having a few drinks.

We went on vacation and I saw a picture she sent of her legs to him while laying on the beach which she says was just an innocent photo.

No matter what the issue there is always an explanation that I seem to buy.

We have talked about getting married and tell each other we love each other.

We have moved in together and most of the time things are good. But I have some trust issues and just wanted to see if I am being to trusting or is there something else that I should be looking into!?