Mammary tumor or mastitis?

[Dream Mama]

I have a 2 year old girl that had a litter of 6 puppies 8.5 weeks ago. She was bred on her second heat. During the time she was nursing, she was VERY prone to mastitis-being treated twice with antibiotics, getting warm baths, and milk expressed. I decided that having further litters was a bad idea for her. I had her spayed yesterday. When I picked her up this morning, the tech comes out and says that doc thinks she has mammary tumors in the last breast on her right side. When I asked if they were sure since she had this history, they looked perplexed (because no one looked at her chart to see she had just had pups). They want me to come back in 2 weeks to recheck this area. I really think it is milk drying up or lingering mastitis. I asked for them to send me home with some antibiotics, to which they replied no...I mean really, what would that hurt? Are there any breeders out there that can offer an opinion? I would think it would be rare for a 2 yr old dog to have this and it show 8 wks after pups. I have the appropriate dose of amoxicillin here and am dosing her anyway.
Thanks!