Mainly because she had a freaking baby bump!!!
But... as much as Bella likes to document her life online, she also loves to play jokes on her fans.
At the time, Bella was filming for her Freeform show Famous In Love, so her pregnant persona was most likely Paige.
Of course, some fans were genuinely wondering if Bella was pregnant, which urges us to explain that if Bella didn't have that ginormous belly the day before, she definitely wouldn't be able to grow that overnight.
She does look totally cute though.