Why does she keep telling me she might be pregnant?

Since I met this girl, every month her period comes late, up to 3 weeks sometimes!
We get tested almost every month and she ends up negative...

After 6 months of this, constantly worrying about it and feeling anxious and scared, she tells me that she's had irregular periods since childhood..

Why would she make me worry for so long knowing that she had irregular periods to begin with?
Why does she keep telling me she might be pregnant?
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