She hasn't been answering your
calls or calling you as often as
before; she's always busy texting
when she is with you can refuses to
let you know who she is
messaging; she cancels dates and always giving reasons and excuses
that you know are impossible; she
suddenly starts to go out a lot more
with her "girl friends" when she
doesn't even like them; she begins
to wear clothes that are more flirty and sexy whenever she is going
out without you; you call her house
but realise she has gone for a
sleepover that she never even
mentioned to you before...is she
cheating?? Please nairalanders help me put taughts in this matter with your own expreince!

Man, I'm not going to tell you stuff that will make you feel better as that will hurt you later. I will tell you the blunt truth.
You don't need to catch her as that doesn't solve any problem in itself. It can only lead to regrettable stuff if you are quick-tempered.
All the things you mentioned are signs that she has lost interest. This does not mean she is seeing someone else by default. What makes it obvious she's seeing someone else is her taking extra care to look more attractive when going out. I've seen this over and over again in different scenarios and it's just the same.
Now, she may not yet be seriously involved, so you will have to be TACTFUL if you decide to talk about the subject(which I encourage you to do) with her to prevent her from playing the lack of trust card. If she is indifferent, just let her go and you'd be so much better off after the initial pain.
NB: I know people will come with the "give her the benefit of doubt", "how do you know she's cheating?" and "trust her" anthems and that's fine. But please nobody should quote me only to talk trash.

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sexkillz!