In novemebr my girlf friend and I had unproteced sex and she took plan b. She had her period on time around the 11 and she had her period on time in december as well. However she has missed her period for january and its now feburary 2 we are starting to freek out we had unprotected sex between novemebr and january but she has taken three test and they all have come back negative is this from the plan b?