Rihanna Smacked in Face by Umbrella Before Met Gala, on Video

Rihanna says when the sun shines, we shine together ... but, when it rains, it's better to stay far apart -- 'cause the singer-songwriter took an umbrella to the face in stormy weather!

The singer-songwriter was out in New York City before the Met Gala on Monday ... and, while marching into the Carlyle Hotel in Manhattan, she came up behind a man holding his own umbrella.

Check out the clip ... the man turns and his umbrella comes under RiRi's own -- which she's holding herself -- and appears to smack her right in the cheek.

It's a total accident, and Rihanna continues on her way ... no harm, no foul -- even if the equipment did nearly take out her eyes.

While from one angle it looks like the man is being careless with his umbrella, another angle shows Rihanna's security guard came up on his other side and touched his umbrella, causing him to turn and smack Rihanna ... so, it's sort of a dual-fault situation here.

Rihanna recovered from her traumatic incident outside the Carlyle in no time at all ... showing up for the Met Gala in the evening -- and revealing her growing baby bump to the assembled crowd.

Our sources say she and A$AP Rocky are expecting their third child ... almost two years after she gave birth to their son Riot Rose Mayers.