Is Lindsay Lohan Behind Amanda Seyfried's Breakup?

Mean Girls drama is spilling over into real life!

First Amanda Seyfried tells Teen Vogue she isn't sure if boyfriend Dominic Cooper is "the one," then rumors started to spread this morning about Lindsay Lohan and Dominic getting "flirty" at a Cannes party last night... and now Amanda and Dominic are "on a break"!

"They [Amanda and Dominic] have been on and off for a long time," a source tells Us. "Right now, they are taking some space from each other." And of course, the source claims that the two "are still very good friends."

But a witness at the Belvedere party at club LeBaron in Cannes also tells Us that "it definitely looked like something was going on" between Lindsay and Dominic last night.

Did LiLo lose her passport AND break up her former co-star's relationship in one night?

Calls to Amanda's publicist were not returned...