why am I such a fool?

Well,as I said earlier he is back from holiday.I spoke to him on the day he got back although it wasn't the joyous conversation I had imagined as he was knackered and seemed to think it would be better to speak the next day-er no not in my books it wasn't.Anyway he's been ill all week so I havent seen him and he wasn't returning my calls.I finally spoke to him last night (!)and he said he had been running between bed and the toilet etc etc,I didnt quite belive it all but he said he'd phone me today.You can guess the rest,I asked him to call me before X-Factor (get my priorities right!)and in the meantime I was browsing Facebook (devils work)and saw a post off a woman saying 'are you excited?' so I can see straight away he's been intouch with someone,not me.So I do a bit of detective work and look at her profile,and see shes bought tickets to something-I'm guessing thats what the post is about.I keep digging and check her posts and they are all relating to him,these are them;
X wishes the sexy policeman would hurry up and get better!
X cannot wait for the policeman to get home!
X is counting down the days until the sexy policeman gets home :(.
X going to start doing some work so that she wont be thinking about the sexy policeman being away for 2 weeks :(.
X thinks that a certain policeman rocks her world!

I am shaking,I keep digging and on his posted items he put something like 'policeman and exGF are such dummies they cant rotate the picture',the day this was posted they are also listed as being in a relationship-this was back in May when we were supposedly trying to sort out a relationship with each other and its also alot later in the year than when he said they were seeing each other which I kind of had sussed out already,at that time he had some pictures of his daughters on there with her and when I quizzed him about it at the time he said it was a friend,he later admitted who she was,but that the pictures were old.

I can almost understand the lying about the ex-GF to make it easy to explain to me,but this new woman????All this was/is happening whilst he was texting me sweet nothings from holiday and joking about future babies before he went.
I am in shock,this is not some random man who I don't know from Adam,I've known him for about 2 1/2 years and have worked closely with him and been close to him as friends.He chased me (see previous posts!)and even yesterday was telling me not to give up on him,on holiday he was saying how much he loved the place and 'next year you're coming here with me'.I am going to let her know what a wanker he is.

Seriously,what do I do wrong?I loved him which was rather quick but the friends thing changed everything.