I think he's cheated..

Sian
Okay, so me and my partner have been going out for 9 months now. I was scrolling through instagram, and came arcross a picture that one of my mates liked. 
It happened to be his ex's sister, so me being me I was looking at her pictures 🤦🏻‍♀️ so I fast scrolling out the corner of my eye I saw a photo with him init. Looked at the date it was a month into our relationship, We got together on 22/06/16 but the picture was dated 8/07/16 what do I do! And to top it all off his ex girlfriend is pregnant, she's saying it's his but his saying it's not and she's due in May so something don't add up!
He's away at the moment doing his army, and I don't wanna message him as he won't see it until his finished in 3 days time. What do I do please help!