Hi Rachael, I had this happen twice. When I had my first mastectomy I had a small bleed after a week or two. It was about the size of a golf ball, and my surgeon chose not to do anything other than have me put hirudoid on it.( anti bruising cream) It did resolve, but it took about 8 weeks. 9 months later when I had my second mastectomy and insertion of tissue expanders, I had another bleed at the 3 week mark. This was much larger, and happened quite spontaneousy. I was about to get on a plane to fly to Sydney from Perth after a brief holiday. I had to have emergency surgery in Perth, where I had 800 mls of old and new blood removed, the expander washed out and replaced, and a new drain inserted. This then drained off another 200 mls of blood. This haematoma was very painful, but it all healed quickly, and I have had no issues with any of it since. My implant is soft and comfortable. You should find that your wound will heal very quickly once all the old blood is evacuated. My small non surgically treated blood clot took so much longer to resolve than my huge surgically treated one.
Good luck with your surgery, and please let us know how it goes.
Love Chris xx