NEW primary in 'good' breast 2 months after chemo?

I am trying not to worry about this and fear the worst... Before I explain what is currently happening here's the back story: 

Was diagnosed Jan 3 this year at 28 years of age with IDC, stage 2b, grade 3, 8/8 ER/PR+ Had a MX with immediate recon on left side and axilla dissection (only 1 node + and was .4mm) Had breast reduction on right side to 'match' left recon. Started IVF to harvest eggs (where potentially estrogen could have spiked temporarily and was put on Letrozole to counter afterwards for 10 days) Started AC/T chemo every 2 weeks April 3 and last treatment was July 10. Had a bone scan BEFORE surgery that was clean, abdominal ultra sound that was all good, had an MRI 3 weeks before surgery that didnt show anything suspicious in my 'good' right breast.

So everything should be OK right now (???) and I am starting 28 rounds of rads next week... but then about 4 days ago I noticed my right boob was sore on the underside of it (near the T section scars from my reduction). And that's when I found a lump there. It feels deep and can feel it more when I am laying down then when sitting up. The skin is red and swollen and it hurts. I have an ultra sound scheduled September 14 and saw my radiation oncologist (pre-rads planning 3 days ago) who inspected it and said it is extremely unlikely that it is cancer because I just finished chemo. She put me on anti-biotics cause she thinks it might be an infection of some kind. But she said the ultra sound would determine more if the drugs dont help it. I expressed concern that it might be IBC because of the redness and inflammation but she said that is just so unlikely and shook her head. She confirmed that there is a definite lump there though. It is about 2cm wide.

My fear and worry is that it is a new breast cancer on my good side. Is it possible it could have started growing after my surgery and survived through chemo and is now showing itself? My last tumour grew in under 6 to 9months. Has anyone else had something like this happen before? Is it possible? I know my RO doesnt think so but she did say it is possible of course but was rare.

I think what is making my fears worse too is that I have had some dull chest pains lately and I am more sore and achy now then when I was doing chemo. I thought being almost 2 months since my last treatment that I'd be feeling better by now and not as achy. It hurts to get out of bed, kneeling in the garden, stretching, bending, getting up after sitting, it all hurts so much! So I guess I am fearing that somehow I have developed cancer elsewhere in my body.... My knees ache like crazy, my hips hurt, joints... Im only 28 and feel like Im pushing 90. I told my MO this last time I saw her and she was surprised but not worried. The next time I see her is Sept 21.

So now I wait to see if these anti-biotics kick in (so far no improvement with the redness/inflammation and its been 3 days and the lump is still very much there) ... I guess I have to wait until my ultra sound on Sept 14. 

Would appreciate hearing opinions/experiences from anyone else who has had something similar. Thanks so much in advance.