Tiger Woods Ex-Mistress -- I Bet He's Already Cheating on Lindsey Vonn

Once a cheater, always a cheater ... so says a former Tiger Woods mistress who tells TMZ Tiger's new GF Lindsey Vonn better know what she's getting herself into.

We spoke to former hooker/porn star/ Devon James -- who tells us she was shocked when she saw the pics of Tiger and Lindsey on Facebook yesterday ... saying, "I thought he would have picked a woman who was more petite and not someone who is more muscular and has a bigger head than his."

James continues, "But maybe his taste has changed since Elin and the 27 mistresses."

"But you know when the pics came out of Tiger and his new girlfriend ... somewhere, 15 other girls were going , 'Huh? I thought I was his GF!'"