Barbara Palvin is getting real with her fans about a health struggle she's kept private -- revealing she recently underwent surgery for endometriosis after years of painful, debilitating periods.

A-listers gathered on Sunday to roast the crap out of Kevin Hart ... but the comedian wanted to get things started by taking a few shots of his own.

Britney Spears' Instagram has been dominated by wholesome, quality time with family since she returned to the social media site following her latest DUI arrest ... and she just posted…