Husband work colleague?

[TRish]

My husband lives away from home thru the week. We have two school going children therefore we did not relocate with him. This is a temporary measure (for about a year).

He has made plenty of friends since he has moved to the new town. I know he socialises with his work colleagues frequently through the week (I have no issue with this).

What concerns me is that one of the women he works with texts him quite frequently. She ends some of these texts with an X. I did not look through his phone for this. Apparently she has a new boyfriend who also works at the same company. My husband continually talks about this woman and her past to me when he is back at home for the weekend. He also checks her facebook account regularly. I am confused and frankly upset about all of this.

I have told my husband how this makes me feel and he has reassured me that he would never cheat.

He also thinks I am jealous because of his friendship with this woman. He has never offered for me to meet any of these new friends

Is this normal within a marriage, for a husband to behave like this?
I also work full time and earn significantly more than my hubby so he is most certainly not providing for me
I did suggest that we relocate there too which he seemed unhappy about initially but now he seems to think its not such a bad idea. Unfortunately, my job is genuinely too good to leave. I feel as if he is also fixated by her, they are the same age, apparently share common interests and have lots of "fun" poking fun at each other. I explored with hubby what he would feel if I suddenly had new male friends whom he had never met and continually spoke about them and if they were texting me. Hubby said the X means nothing (apparently all women do this to their male friends (i dont))
I know that he looks at her facebook continually as he looks at it before he goes to sleep in bed. I have sneaked a peak when he was laying in bed on his facebook on his phone - he thought I was sleeping. Interestingly, he now deletes his internet history - all of the time. I had seen a search for "good sex positions". I know for sure that this is new behaviour. Hubby texts me everyday to say that he "loves me"
Hubby also mentioned that new female friend wanted to chat to me on the phone (about our relationship!) i politely declined and was extremely upset
any further advice out there?
You say its an affair? do you mean an emotional affair? or a sexual relationship?
Do any of you constitute this behaviour as an affair? and what should I do about it? Thank you