Given that it is not rising very much, I would guess it is retained placental tissue. It's the placenta that produces the hCG reading, so it can go on for a while if a little bit of living tissue was still attached to the uterus.
As long as the hCG hormone is in your body you'll have pregnancy symptoms. So tender breasts don't actually indicate too much--you'll have it regardless of pregnancy viability or not. As for your levels dropping and rising, it could be due to a vanishing twin or retained tissue. Get an ultrasound from your doctor and have him check your blood work again. If it shows them dropping and/or rising slowly, you'll likely need a D&C done.