How do I proceed with this one

I have found the following on my husband's facebook private messages. I will refer to him as H and the lady as T.

H: Hi its me H, I am registered as .....on facebook.

T: Hey Lover

H: Me and some mates are going to ...country in Europe ( we live in England.) Would you like to come along? All expenses paid

T: Will I be the only female

H: With me yes, not sure about the other guys.

T: Sounds like fun

H: The guys are getting excited about the red light district.

End.

We have been married 8 years, no children. my husband told me about a trip for a birthday's friend some two weeks ago but he said he will not go because I am pregnant and I have a history of difficult pregnancies. I however cannot understand why he was chatting with this woman who he claims to have had not contact with since they worked together over a year ago and she is just a work mate. Why was he telling her about the trip, he did not comment when she referred to him as lover and the whole thing is very upsetting and suspicious. When I asked him to explain, all he says he is not having a relationship with this woman. He showed me her facebook status which says she is in a relationship with someone else.This to me does not mean anything I am very upset as he will not explain to me why he was having a chat of this nature with this woman.He says he does not know why she called him Lover and there is no reason why he mentioned the trip to her. please help.