Positve pregnancy result one day then negative the next with no bleeding and pregnancy symptoms

Patient: I am taking Clomid for fertility, my period is late by 6 days, so yesterday i took a home pregnancy test and got a positive result, i have had 3 ectopic pregnancies in the past so went to the early pregnancy unit today, they scanned me but could not find anything, so they did another urine test and told me the result was negative, i am having symptoms of pregnancy, sickness, sore breasts, headaches, tiredness etc, i just want to know how i can have a positive result one day and a negative the next when i have had no bleeding so havent miscarried?