Faint positive then negative straight after

Hi so I do tests every day, I'm 30 days late according to this app, however my periods are only just getting back to normal after my ectopic and MMC, I buy these in bulk from amazon. I went to loo like normal and peed on the dip stick mid stream- faint positive. I then did the second dip stick dipped in my next wee however this came back negative. Iv no more tests so ordered another batch from amazon however why do you think this has happened?

Thanks