Do you think she noticed?

I wondered what most guys would do in this situation,
Or if girls would read into it? If you were sharing a bed with a girl (that wasnt your g/f) and you used to be attracted to them/had at least kissed, in the past, would you find it difficult not to be aroused? (me and this girl had nearly- slept with eachother in the past, we did stuff but she didnt want to go that far, though I did I stopped. Anyway this time (she was wearing a long tshirt of mine, underwear but no trousers. We were right next to eachother/our bodies had contact nearly all night. I turned my back towards her mostly but do you think that made it more or less obvious that I had a erection? Wouldnt letting her into my bed in the first place suggest im comfartable doing that without being sexual? Or did it just give her a bigger signal? (I slept in bed with her a week before) would you have done the same? (turn your back I mean)
P.s atm I am in a relationship with another girl

Umm would you think your g/f was cheating if she was sleeping with another guy with an erection and she was only wearing a T-shirt and underwear? Com'on dude look at things on the opposite side of the spectrum.

More importantly, why are you sharing a bed with another girl when you have a girlfriend? If you dont believe in commitment, then your girlfriend really doesnt deserve you! I dont think your question has any meaning, except for the fact you are cheating, and all you care about is having an erection in the presence of another girl! I personally wouldnt have any thought one way or another as to why a guys back is facing me! I mean, who cares! Unless he was my boyfriend.

Agree'd with the comment above.
If you have a girlfriend you shouldn't be sharing your bed with another girl. If my boyfriend shared a bed with someone he used to like, I'd flip.

All very true above.
I think worrying about this girl noticing your erection should be the least of your worries. You should be worried about yourself and be feeling pretty bad about cheating on your girlfriend. You should be talking to her and sorting a few things out.

I didn't cheat on my girlfriend since when was sharing abed with a FRIEND cheating? what the f*ck :S