Can someone help please?

Tyisha • 20years Old
My mind is on overdrive... I'm 20 years old , my boyfriend is 21 we have been trying for a child again after a miscarriage.. But I was on my Instagram and my Facebook yesterday and under suggestions or people you may know I saw a familiar face ( my boyfriend) with another girl loads of picture and collages of them two and loads of captions as our night together or our anniversary ... So I confronted him last night and he said it is his ex and he begged me not to message her and not to tell anyone because he doesn't want his family to see it because they might think wrong but he strongly wants me to believe that she is a jealous ex and she is obsessed with him?? I don't know if I believe it though?? I just hope us trying for a baby I don't look like a mug if he is cheating on me. 
What do you guys think? And what would you do?