Is my sistee pregnant?

A month ago my sister had sex with her boyfriend he claims he came inside her but her period came on 2 days early and it was heavier than usual, however she thinks she is pregnant due to the fact she’s sleeping way more, she’s eating way more she literally just ate and she’s hungry all over again, she took pregnancy test but they are negative my sister eats big meals and still be hungry and sleeping way more what is going on? Her COVID-19 test was negative we just need answer before we go to the doctors