TTC but is he cheating?

I've been with my boyfriend for over two years. We've been living together for 1 Year. He was actually the one who bought up TTC and said he wanted to start a family. We had a lot of chats about it before starting to try. Last month we had a chemical pregnancy at just over 5 weeks. he was so supportive looking after me and said athough it was sad it proved to him how much he wants us to have a family. 

however now I'm having doubts..

He started a new job recently and a support staff at his work is female. I have no issue with this I work in a male environment. However. She started contacting him a lot outside of work. A lot. Facebook snapchat texting. He started deleting the messages and messages received would be the first one when I knew she had to contact him about work things. That's when alarm bells started. A while later I accidentally saw a conversation text between her and him. I wasn't snooping he read a text while I was next to him. It said;

Him- I'm sorry but no. I can't risk it. I have too much to loose.
Her- no worries I am a sexual (a nun) anyway.

we spoke about her contacting him I asked him to stop outside of work but understood she would need to contact him about work things.
Last night he was next to me and got a snap chat. Without meaning to I noticed her name in his list of recent snap chats, he has been in contact with her on snap chat. They have a streak going of several days and it shows them as best friends with the icon.

Now for why I'm struggling to bring this up. His nan has just passed away last night. So as I was about to bring it up he got a phone call that caused him to naturally be extremely upset. It obviously isn't the right time to bring it up but it's bothering me so much it's all I can think about.

I am currently in the tww for this month and unsure about pretty much everything.

Any advice???? Please!!