Mammary tumor, or mastitis?? Help?

if you're concerned and you don't feel this is something that can wait go ahead and take her in. mastitis usually occurs after a recent litter of pups. a vet is going to be able to tell you what it is exactly and how to best help the dog. warm compresses are the best thing you can do right now. she will more than likely be fine until morning, but again if you're worried i would go ahead and take her on in now, the er vet can at least give her some pain meds and tell you what's going on. good luck and hope everything turns out ok.