Breast generally become somewhat swollen during estrus because of the hormonal changes and the tissue is likely more fragile. Maybe this could explain the bruising. Also you said the back 4 nipples-might they be traumatized from banging against her legs when walking/running? You might want to check to see if her breasts are warm or hard to the touch, possibly indicating mastitis. A little early for a false pregnancy but as her seasons seem a bit off so might the time frame. Of course I know you'll check with your vet if your concerned.