Does it take a while for back pain to go away after a breast reduction?

While some patients note an immediate improvement in the heaviness that they were experiencing before breast reduction, it can take some time to see improvements in symptoms including pain in your back.
Years of poor posture can not be expected to resolve in 3 days. Concentrate on recovering from your surgery and I am sure you will feel an improvement in your back pain as well as improvement in your posture as time goes on.
Congratulations!

Thank you very much for your question and
congratulations on your recent breast reduction. At three days post-op, you are
still in the early stages of healing. There will be some level of discomfort,
although this should resolve over the coming weeks. Back pain, caused by the
weight of your heavy breasts before surgery, will take time to improve. If you
have any further concerns about your back pain, or notice an increase in
discomfort, I recommend contacting your operating surgeon for their advice.
 

If the back pains related to the weight of the breasts, it would likely take a few months for  it to improve. Part of what you might be experiencing now is positional when you are lying in bed.