I'm a married man with a 3 years old daughter.
I suspect that my wife is having an affair with a colleague of
hers.
I realized that since several months ago, she has been spending
a lot of time with her male colleague. He sends her to work and
send her home from work everyday. She comes home late every night
and claims that she needs to work overtime. She also goes out and
only comes home late at nights or early mornings on weekends.
She spends little time at home and we seldom have time to talk.
Her work also requires her to travel overseas frequently and i know
that on many trips, she went with that colleague.
I'm at lost now and not sure what to do. Can someone give me
some opinion or advice.
It must be
hurting when you have strong grounds to suspect your
wife having an affair with another man, especially when
the logical flow of events reinforces the notion of cheating. There
are many ways to know if she is having an affair, however, somehow
that is still not my primary concern. Be prepared when you
decide to pursuit what you probably felt as 'the truth' behind the
scene' because what I am concerned about is the
aftermath.
What will happen after you corroborates your suspicion
with facts?
Structurally, the relationship has all factors pointing
towards some dire breakdown; nonexistent communication
and apt towards an individualistic
lifestyle. In fact, the start of the marriage was
somewhat 'coerced' - with greater consideration towards
the unborn child before marriage.
You may be ready psychologically and emotionally for
your marriage, but this remains a question for
her. And albeit she may be ready for
the marriage, she might not be ready to be a mother
and lead a family-centric life.
Agreeing to be married does not answer this question; just
like pregnancy does not automatic make mothers out of women.

***
You are entitled
to know the truth that veils behind her seemingly 'busy' lifestyle
- but before you go about confronting her and ask honestly (or
hire a PI); you might want to assume the worst scenario
(which is yes she is cheating on you) and reflect on
why this marriage has gone wayward because it
would help you to craft out the content to engage a HTHT
with her to see if this marriage is still
salvageable.
P.S: A perfectly satisfied woman cannot be seduced; surely there
is some unmet needs that is being fulfilled by the other guy.
Knowing/confirming that she is seeing/being close (physically or
emotionally) to another guy does not provide you
the insights of what went wrong with your
marriage.
You know that you have not been talking; make that happen and
see how it goes. 
Cheers