OK I'll try and be brief. 
New collegue started workforce end of last year.
We chatted lots and he messaged me which often ended up often just chat about general shit - films etc. Conversation then got more intimate, fears, future etc.
Met up one evening with my DS as he was looking after someone, was just mates etc. We really clicked and arranged to meet up at pool one day as I was starting to swim again and he uses gym. Had a really good time. Previous to this he has been saying really complimentary things and is one of these people who's quite astute so seemed to have me sussed iyswim? Scary but nice at the same time!
Then he popped over one evening and we had a great time, real laugh and ended up cuddling on sofa and kiss on cheek on leaving. Came round few days later cuddles and lip kiss on leaving. After that it was getting heavier but we both said we really didn't want to complicate things as we are collegues and that we would be friends. There was no intention to get more intimate but it was happeneing and we did laugh, talk about it. We were very open.
Things kept going and we had sex a few times.

Now we communicated well through this and appeared on the same page. We continued to chat loads. Agreed to keep it as friends.
Now this is where I am stuck on what to do. It seemed as though he was cooling off towards me and we were having more of a friends relationship which tbh was easier. (not easy facing someone at work a night after sex and pretending nothing has happened!) Then suddendly I got a very caring message after he'd seen me chatting to a collegue and had been upset. I said all fine just having one of those days but thanks for asking etc.
It just then got to the point where he seemed like he couldn't be bothered to reply, or short replies if I messaged him and he didn't really try and contact me.
Now though (2 weeks later) it seems he's finding things to talk to me about at work?
I really did start to like him but always knew he didn't want more because of work situation. I did believe him though when he said we had a good friendship.
Part of me really wants to ask him why the sudden cold shoulder because I feel like a mug and he lied to me but part of me thinks (in a way knows) it's not the cold shoulder but can't work out what it is?
So should I ask and how should I word it!
BTW I am not as childish and needy as my post comes across but having been single for 6 years! I really am out of touch on what to do here and the etiquette if you like!