I would be concerned that she is having a pseudo (false)pregnancy and that her breasts are swelling as a result of milk production or hormones. This is common in dogs.
If this is not the case, she might have mammary tumors but it's not very common in dogs this age.
No matter what, you need to see the vet for a fine needle aspiration if they are lumps. This will rule out a malignancy.
Let me know if you have more questions. I fear that my answer is too vague. If you still have specific questions, feel free to ask.
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