I suspect my wife is cheating.. What do i do ?

I have been married to my wife since march 2012 and we have a 3 year old boy.

i would have described the marriage as so far so good  but Since about 3 months ago i have been suspecting that my wife is up to something but i cant place a finger on it.

It all started when she started becoming very protective of her mobile phone and having frequent mood swings when we are together.

She switch her phone to the silent mode when she is at home and frequently either doesn’t pick her calls or walks out to receive the calls. This shouldn’t have been a major cause of worry but my intuition increased when i realized she had securely passworded her phone – which is unusual.

Her attitude as well has become edgy and more irritable. This has made her more distant and unapproachable

I have subtly inquired from her what is going but it she snaps at me that there nothing wrong with her.