The problem with whatsapp is- a whatsapp camera necessary means your gallery.
And your gallery, if not chosen otherwise, saves stuff according to date and time.
Taking a screenshot in a smart phone is an easy but a tricky manuever. You cannot even take it with one hand. You definitely need two hands for precision (avoiding hitting other buttons). In Samsung phones for example, you take a screenshot using both power button and home button.
I need to catch my phone with my left and do the manuever with right.
In phones like Redmi, mostly there's a screenshot tab giving on the "slide bar".
In most Android phones it is using lower volume and power buttons.
You might have understood by now that accidental screenshots are a rarity.
But if you somehow accept that the screenshot was accidental, it ended up the first in her gallery.
After a time, she might have thought of changing her dp, which according to you didn't go well for you.
We can only guess that she blindly tapped the first pic,I.e..; the screenshot , and ended up as a dp (profile picture).
The stronger a light shines the darker are the shadows around it.