Nipple problems are very common, and since this just started today, I don't think you need to be concerned yet.  Your bra or sexual activity may have caused some chafing and irritation.  If it gets worse, or if it hasn't gotten better in a week or so, it will be time to contact the doctor about it.  I'm not quite sure what kind of chest and back pain you are having.  If you might have done something to pull a muscle, then you could wait to see if it gets better on its own before calling the doctor.  If you are having trouble breathing or if your heart is beating in an unusual way, you should call the doctor right away.  Is there someone who can help you evaluate how serious this situation is like a parent or older friend?  There is no point in delaying seeing a doctor if you are concerned that the problem is serious.  Delaying treatment could just make the situation worse.  We are not doctors here, just well-read breast cancer survivors; however, I can reassure you that it is too soon to panic about the nipple pain being related to breast cancer.