Serena Williams -- Butt-A-Like Blowing Up ... After IG Shout-Out

There is money in Serena Williams' butt ... or in having one that looks exactly like it ... 'cause an L.A. model says that since Serena shouted her out as her official butt-a-like ... her phone has NOT stopped ringing.

The model's name is Olivia Griffian, and she tells TMZ Sports her life was changed after Serena posted a picture of the two standing back to back at a recent shoot with a caption that read, "Found my Doppelgänger which one is me?"

Olivia says after the post, her Instagram followers nearly doubled and she was inundated with calls from agents and managers wanting to rep her ... all because of her striking resemblance to Drake's boo.

It's no accident that she was body doubling for Williams ... she claims the two are friends and talk everyday ... but she never thought looking like her pal would be worth so much fame.

Check out the clip, and the gallery below ... she might actually give Serena a run for her money.