Text message confusion

[BEN.KING]

I have recently been having problems with my girlfriend, well, ex girlfriend now. I hadnt heard from her for about 3 or 4 weeks and I thought she had just gone off me and was ignoring me. However I recently managed to get hold of her and I asked why she was ignoring me. She told me that she had recieved a text from my phone which was supposed to go to another girl, which obviously made it look like I was two timing her. I was completley stunned by this and I really didnt know what she was talking about. However around the time she got the text my phone was in the O2 shop being repaired. Anyway I have tried endlessly to get her to believe me, that I really did not send this text, but ( to be honest understandably ) she just doesnt believe me, she has told me to leave her alone and delete her number. I'm totally gutted because this has been some kind of error or something. There was nothing in my sent box when I got my phone back. Does anyone know how this could have happened? Is it someone playing a sick joke or could there be a technical explanation?
I did ask this in the relationships forum first but I thought I would try in here in case any technological experts might have any idea what has happened, oh and I did keep my sim card when the phone was in the shop

[Twirls]

Wasn't this on yesterday or did I imagine it?

[yelenots]

Yes I have read before too

[BEN.KING]

Yeah but I thought I would try it in this forum incase any technological experts might have any explanation

[AlBags]

You don't need a technological expert mate ..
You need a new phone!

Then you can pretend you had it ages ago and flogged the old one.