Thank you for posting your question here, I will answer it as best I can.
You've read correctly, yes it is normal to have a brownish discharge after intercourse. When is your next period due? It could be that your breasts are feeling sore because your period is due soon.
Another possible reason is that your boyfriend was a little too rough with you, that can cause sore breasts as well.
But in the future use a condom at least, it will save you a lot of stress
So yes it is not likely that you are pregnant, but retake another home pregnancy
test in around 4-5 days and that should give you the answer you need.
I hope this answered your question.