Lurkette- do you have the hormonal or non hormonal one? If you have the hormonal one I'd say that could be causing it, if you don't have the hormonal one then I'd say it's probably something else. Remember that sometimes your body just does strange stuff, and you're like "hrmm... that's wierd" but there's not really a good reason for it. I know that I sometimes have random spotting between periods and I've had my IUD for over a year. According to about.com, IUD failure rates are second only to tubal ligation, so I really wouldn't worry about being pregnant.
Take a pregnancy test if you're concerned, and if that comes up negative go see your OBGYN. Breast tenderness sounds like body wierdness, weight gain might also be body wierdness, back pain could be tight muscles. Who knows? *pats lurkett on the back*
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Sage knows our mythic history, King Arthur's and Sir Caradoc's
She answers hard acrostics, has a pretty taste for paradox
She quotes in elegiacs all the crimes of Heliogabalus
In conics she can floor peculiarities parabolous
-C'hi