There is nothing worse than being excluded and most probably you have not done anything wrong. Maybe it is time you stopped thinking of them as friends, and perhaps made an effort to be friends with some others at work.
As for asking them about it, they don't seem worthy of the effort, their behaviour seems childish, but when they lie to you, you could say something like 'you don't have to tell me where you've been it is ok you know, I am not the boss' but do it in a friendly way with a smile on your face like you just don't care, because you shouldn't, just keep it light hearted.
You don't get to choose your work colleagues some do turn into friends and others do not. And get out away from your computer at lunch time even if it is just to sit elsewhere or go for a walk.
