BFP then BFN

TTC for 6 months with first pregnancy. On cycle day 34, I got what I thought was AF but may have been IB due to it only lasting one day. On cycle day 37, I got a BFP using clear blue digital. Scheduled a confirmation test with doctor on cycle day 42. In mean time I tested again using clear blue digital and got a BFN on cycle day 39. Any thoughts or feedback?? I'm very confused and have had no symptoms or changes.