I have an open wound on my breast. I just got a breast reduction a month ago, what might have caused this to happen? (Photo)

Open wounds are not uncommon and is usually caused by skin tension due to the added volume on the breasts. This is easily treated with topical antibiotic creams and dressing. Consult with your surgeon regarding wound care and make sure to prevent infection by keeping the wound clean and dry at all times. 

Minor wound problems are not unusual following breast reduction, especially in the vertical scar, or at the T junction. This is the area where the greatest tension occurs in a reduction, and the skin / tissue is the greatest distance from the blood supply (which comes in down the inner and outer parts of the breast. Looking at your photos, the wound is gaping a bit. It will heal in the end, but that will be fastest witht he right dressings (sometimes a 'negative pressure dressing like a PICO can help). It is certainly worth your PS taking a look - but be patient, as it will heal in the end.