I've only just seen this YG - it must be v stressful .
I don't know if the following helps, but this is what happenened with a friend of mine, when, at 6 months pregnant with her first child, she began to worry that her dh might be having an affair.
Firstly, she took the approach suggested by tortoishell's dh, not accusing her dh of anything, but making sure he knew how distressing the situation was to her etc. He denied categorically anything was going on, she was a nuisance, didn't know what to do to get rid etc etc. She suggested he ask her out for a drink, pretending it would be just him turning up, when in fact, she would turn up too, nothing threatening, but a "hey, dw decided to come along too, isn't that lovely !". Her dh came up with every reason in the book why this was a bad idea. From then on, although she hated it, she turned detective, monitoring his movements and his mobile.
Unfortunately, she found out he was having an affair (text on his mobile which she found when he was in the bath left her in no doubt what had been going on).
I hope it all turns out to be just someone who is a PITA who can't take a hint.