The vaginal muscles often relax after orgasm - doesn't mean she's sleeping around, she might be masturbating. Vagina's, like penises, come in different sizes, and even a lot of sex should not have any permenent effects - in fact women who don't orgasm often enough are at risk of developing what is called "senile vagina" in their later years - orgasm causes spasms, which actually exercise this particular organ.
She might just be very large - if you feel like your experiences with her are satisfactory, you might consider learning to fist her if she's that loose - go very slowly, and I recommend researching it thoroughly - there is a remarkable range of articles on the subject on the internet, search vaginal dilation too - but generally, the techniques are similar to those found in the anal sex thread - and there is often even a tightening effect afterwards, so powerful can the resulting orgasmic spasms from this be.
I pretty much guarantee she'll quit health forum about how small your dick is.