Positive hpt then a negative 3 days later?

hello, so i had what i assume was decidual bleeding. it was more than spotting, less than a period. anyway, on thursday i took a ht after the bleeding stopped andgot a positive !!!! then now on sunday i took one and got a negative. has this happened to anyone? i dont think it was a chamical pregnancy because i hada positive after the bleeding. someone please offer some reasons