If there is no swelling or redness of the actual breast then it can probably wait till the 11th. But I would mention this to the vet. It may well be hormonal, but my dog used to get this at certain times during her heat cycle. She was "entire" too, and had borne a litter in her early years. She sometimes would get that brownish discharge slightly from one large nipple, yet at the time had no lumps or swelling.
Then about 4 years ago she developed a small round lump near the affected nipple. Needle-biopsies showed it to be benign, so we opted to leave it.
She was then spayed 2 years ago. However, the little lump grew, and when it was removed (February this year) the lab. biopsy showed that it had become partly cancerous. As a result, her lower mammary tissue was completely removed March 27th.
Now she recovered well and is still very well. I am very glad.
I'm not saying all that to scare you, but just to make you aware that's what happened in her case, and what can sometimes be behind discharges like this. If a small lump is discovered which might be causing this discharge, then my advice would be (benign or not) to opt for its removal.