Is this an infection after breast reduction?

Most infections following breast reduction surgery occur in the immediate post-operative period.  Occasionally infections occur late in the patient’s recovery and this is usually related to a stitch abscess.
This problem occurs when infection develops around one of the absorbable internal sutures that are used during breast reduction surgery.  This can result in a marble sized abscess or granuloma that requires drainage and treatment with antibiotics.  This is a relatively simple problem to deal with if treated early.  If diagnosis and treatment are delayed, then more significant problems can result.
This problem requires follow up with your surgeon.  With appropriate care, this should represent a minor setback with no long term consequences.

Great question. It is very odd to have the presentation you had, as typically an infection etc. is unlikely to present that far out from surgery. Most likely, since it is your incision, is a reaction to one of the sutures that your body had an inflammatory response to. This can lead to an infection or simply be inflammation. It's possible you had it for a while and it finally erupted through the skin just recently. Either way it should get better after drainage and go away.
-Dr. David Mattos, New York Plastic Surgical Group

It is unusual for infection at 6 months but if you had a small cavity with whats called a seroma/fluid filled cavity or a repeated reaction to the suture used (some times seen in any wound closure with any number of suture materials) can lead to what has happened to you and this is the most likely scenario. 
And I want to take time out to put what I'm about to say into perspective because there is no cause of undue concern here-naturally, your surgeon would have stepped back and looked, examined and investigated a new lump in your breast appropriately as you would with ANY new lump in the breast in ANYONE presenting in our office- your cause appear to be obvious in the first instance(but warrant an overall diligent investigation by a specialist of course) because in Australia a woman has 1 in 8 chance of developing cancer in their life time and these numbers are not too different in the US. Such thought is always in the back of our minds as plastic surgeons because unfortunately we do see some rare and unexpected cases. 
Again this is not to cause anyone any undue concern and off the topic except to say that regular breast examination/screening as per your local guideline that is tailored to you and your age and the risk that is specific to you is vital and something none of you with breasts should ever neglect (for example these examinations and investigation if needed needs to be more frequent if you have a family history of breast cancer or are known to have genetic mutations such as BRCA-1 or 2 gene among others that predisposes you to a life time higher risk of developing breast cancer). 
In my practice if you have no family history and no other risk factors but you are above the age of 35 I  get a base line mammogram (some X ray pictures used in screening). Then I get another one after the breast reduction when the wounds have completely settled down just so that I have some comparison in the future should I ever need it and this is done because once I've rearranged you breasts in a way I've "reset" the breast tissues. Interestingly though, breast reduction reduces your risk of developing breast cancer because, and this makes sense, you have less breast tissue now that you had a reduction, therefore you have less breast tissue that can turn into a cancer.
Wish you the best in your recovery.
Dr Leo Kim, Specialist Plastic Surgeon in Sydney