Rebecca threw on a bikini and soaked up el sol Monday in Playa del Carmen -- and it's crystal clear she's living her best life in the coastal resort town. Looks like she was hanging with a friend who understands the fine art of chillaxing through a crisis.

TMZ broke the story ... Rebecca filed for divorce from former "Grey's Anatomy" star Eric just 2 days after Valentine's, and after 14 years of marriage. Eric and the former '90210' star have 2 daughters, ages 6 and 7.

She's asking for spousal support as well as joint legal and physical custody.