Lindsay Lohan -- Bikini Pics Reveal Nasty Bruises

Lindsay Lohan threw on her bikini and hit the beach in Brazil yesterday ... revealing a series of gnarly bruises down her left leg.

It's unclear how Lohan got her new marks ... it's possible she just banged herself up while ducking under tables at a nightclub last week.

Then again, it's not impossible to think she got into another late-night altercation -- just ask that psychic she allegedly socked in the face a few months ago.