Mercedes "MJ" Javid swears she hasn't been photoshopped, she really did drop 20 lbs., and she tells TMZ ... she did it the old fashioned way -- lots and lots of sex ... and some traditional exercise.

The "Shahs of Sunset" star says she's seen all the haters accusing her of doctoring the pics from a recent swimsuit shoot -- but says her real secret is simply banging the weight away.

Best of all ... MJ claims the more pounds she shed, the more sex she had -- and the more sex she had, the more calories she burned. The least vicious cycle ever! 

She admits cutting back on hard partying and late-night boozing definitely helped too ... along with hiking, biking, tennis and dodge ball -- but says she's definitely sticking to her sex diet.  She says bluntly, "I'm too pretty to be fat!"

Her ego is NOT slimming down.