What Treatment Is Suggested For Wegeners?

My daughter is 19. Has irregular periods from the beginning. She has a rare disease called Wegeners. She just finished 2 rounds of Rituximab IV in August and has not had a period since then. Her immune system is low and is doing IVIG treatments to bring it back up. At the doctors visit today he did a blood pregnancy test and it showed positive. She says she is not pregnant. What would cause a false positive with the stuff she has had since August?