Deaddei, I realise you're trying to be helpful - but going for a walk is only likely to be possible if her periods are 'not especially painful' like your DD. If they are as bad as the OP's DD describes, she probably isn't going to be capable of walking.
I feel a little bit sad having read your post, as I went through this too, and it was tremendously, tremendously painful, and it did turn out to have an underlying cause. There is mounting evidence that the proportion of teenage girls with endometriosis is the same as or approaching the proportion of adult women with it - estimates vary from 4-15%, depending on which study you read, and 50-75-ish% of teenagers with severe dysmenorrhoea, also depending on which study you read. That's a lot of teenage girls. But the diagnostic delay is much, much longer in girls who start to have symptoms very young, precisely because people are very ready to put it down to them being 'drama queens' who 'wallow in self-pity', and assume they are doing it to get out of PE. So I do think you're being a little harsh, though I can see you don't mean it, but I think it's worth remember ing that mild or moderate period pains are not the same as severe ones.
I don't see any reason at present to assume that the OP's daughter is putting it on, as what she's saying is pretty standard. And if she is, it's better to assume first that she isn't and rule any possible causes out, because much more damage can be done by assuming she's exaggerating when actually she's not.
This site is useful www.endometriosis.org/teenagers.html
And the presentation on 'Current management of Endometriosis: follow up in teenagers' is useful and gives the diagnostic delay in teenagers as a median of 12.1 years, compared to just over 3 years in women who first get symptoms in their 30s.
www.endometriosiszone.org/display.asp?page=expert_teenagers# (the first link on the list)