Patrick Schwarzenegger -- Miley's Worst Nightmare ... New Spring Break Pics WITH HIS EX GIRLFRIEND!!

Patrick Schwarzenegger's just doing what every guy does on spring break, but in these new photos he happens to be doing it with his ex-girlfriend ... which will be harder to explain to Miley Cyrus.

The pics just surfaced from the same day he was doing body shots off his BFF's girlfriend ... and relatively speaking, they're pretty innocent.

PS and the green bikini hottie -- named Taylor Burns -- are chatting, laughing, and she was even considerate enough to rub suntan lotion on his back. The sun IS hot down Mexico way.

Here's the thing ... guys and girls socialize on Spring break -- sometimes up close and drunkenly. Patrick's just having a regular college experience. His challenge is Miley didn't have that experience.

Still, we gotta say the ex-factor does complicate things. Good luck, Patrick.