Prince Harry -- PHOTO PARANOIA After Naked Vegas Romp

Prince Harry realized he made a huge mistake by allowing girls with cameras into his Vegas hotel room Friday night -- because just two days later in Sin City ... he was INCREDIBLY camera shy, refusing to pose for any pics with fans.

TMZ spoke with one of the girls Harry was hanging out with at a pool party Sunday night -- two days after the naked pics were taken -- and when she asked him to pose with her and her friends, Harry politely refused, saying. "No photos please."

It's a big shift in behavior from Friday -- when Harry invited several strange girls into his Vegas suite, and stripped naked in front of their cameras.

It didn't end there either, Harry happily goofed around at Wet Republic the next day -- probably the most photographed pool party in town.

But by Sunday night, Harry had grown tired of all the cameras.

As for his security ... our source says Harry was SURROUNDED -- in fact, she noticed at least 10 handlers around the pool. According to her, one of the security guards was even in the water with a waterproof cell phone (for aquatic emergencies).

But photos or no photos -- our girl says Harry was really flirty ... saying he was glad she made it to the pool party, and nodding to her bikini.

Harry, you dog.