Hi Jen - Since IBC wouldn't typically respond to antibiotics, as yours did, this probably was, in fact, an infection of some kind; it's possible to get mastitis anytime, not just when you're nursing. Ditto cellulitis. Also, IBC wouldn't result in those more systemic symptoms – aches, fatigue, etc. However, if you want to rule out IBC entirely, your mom is right - a biopsy is the only way to do it.
We're not medically trained here, simply well-read breast cancer survivors; so we can't offer you medical advice, only a shoulder to lean on as you navigate through this. It's up to you to assess your level of worry; are you willing to wait and see if everything simply goes back to normal? If so, will you still be worried there might be IBC present somehow? Or would you believe this was an infection that's been treated and cured? It's a tough decision, I know, but you're the best person to make it; it's your body and mind, and you're the one ultimately responsible for your health, both physical and emotional. Best of luck as you move forward - I hope everything continues to heal uneventfully. PJH