Late and unsure

Fe
So I obviously use this app to track my period and last month I was late by 1 day and that's not so bad but this month I'm late by 5 days.
I took a pregnancy test and it came back negative in the second day 3 days later still no period, I'm not than a little freaked out and don't know if it's common that my period is changing so drastically or if this is definitely something to be worried about. I do think it harbours a visit to the doctors but until I'm able to get an appointment I'm just asking for personal experience, advice just something.
Thank you in advance,
F xo