I had hot flashes and then times when I just couldn't get warm enough in the weeks following my son's birth. I chalked it up to hormomes or the epidural/morphine from the C-section. They lasted for a couple months. I don't know about the lump in her armpit. That definitely needs to be checked out by the doctor. I wouldn't stop breastfeeding for those reasons unless the doctor thought it was necessary. Good luck! Keep us posted!!
Sounds like a possible clogged milk duct (it is amazing how far back those buggers go) I am surprised she doesn't have a fever, given the chills/sweats you have described. Get her into a clinic/urgent care type place. I know it is tough without insurance, but this can develop into something pretty major if left untreated. Most places will work out a payment plan if that is an issue.
I have had a lump under my arm for the last month and a half, i'm 24 weeks pregnant and have seen a Dr about it. The Dr thinks that it's a blocked milk duct, i had an ultrasound done on it and it showed nothing. My sister also had the same problem after having my nephew, her's was a blocked milk duct as well as she was nursing. The hospital put warm clothes on it to see if it would help, it did help and she seemed to nurse a little more as well. It should go away if she continues to nurse and have the baby drain her breast, if you both are still worried about it go to see a Dr ASAP.
I had a lump under my arm with my dd as well. My doctor also said it was a clogged milk duct. He told me to have the baby drain the breast well, use warm compresses and masage gently.