I had unprotected sex at the end of December on the last day of my period. I used plan B immediately afterwards, and a few days after I bled for about 3 days.
Around a 10 days ago, end of January when I was supposed to get my period, I got brown discharge. This I sstill going on and is at times a bit heavy, and I really don't know why. I took a home pregnancy test and also did a blood test and both came bacck negative.

Should I be worried? What could be causing this?

It may be that the plan B caused your cycles to be off. See what happens when you would expect your period next month.

If the discharge continues, you could always check with your doctor.

Plan B often causes the period to be off