positive but period

Why would I get a positive test but then I start my period a couple days later 😡😭 
Update: I called they said it could have been a chemical pregnancy/ false positive. Since I didn't have any cramping or pain it couldn't have been a miscarriage. This was full blown period. I had to change my underwear twice in a day Bc I bled through my pad(tmi) but yea. Thanks!