Bfp then a Bfn what's going on??

Honey
OK so I had my last period on June 25-28th I took a test on the 15th of July I was in shock it said pregnant so took 5different ones all said yes well I decided yesterday to take another and this time I got no but haven't had a period what's going on? Even went to ER and they said I wasn't how is this possible what's going on someone help