To test it, wet a scrap of white fabric and place a damp piece of red on top of it then set a glass full of water on top (this simulates what will happen in the seam). Leave it sit for about an hour -- if there is color transfer either use Retayne or ditch the fabric.
If you remember who you got the fabric from, definitely let them know.
I have had this happen with some very expensive hand dyed fabric and some from Joann's -- I contacted the person who did the hand dying and she sent me a new piece that she treated (she said she was doing so many pieces for the show that she probably missed the piece I bought). I was able to stabilize the Joann's and it has not bled since.