Lady Gaga -- BACK in Bikini Shape

Less than a year after gaining 30 pounds, Lady Gaga is back down to her fighting weight -- surfacing south of the border in Mexico this week with some friends ... in a sexy floral bikini.

You'll recall, Gaga struggled with her weight last year, telling Stylist Magazine, "I don't really care if [people] think I'm fat, because, quite honestly, I did gain about 30 pounds."

Gaga also underwent serious hip surgery a few months ago ... so that probably helped with the weight loss too.