Bounced Project To MP3 Levels Are Hotter?

Hello, 

Working in LP9 V 9.1.8, early 2008 Mac Pro, Universal Audio Apollo interface: Producing a 30 min music show for a local radio station here. All my levels are fine / peaking at 0dBFS max throughout the project. I do spend time adjusting all the tracks (two music tracks and a couple of vocals) to make sure everything is not clipping during the show. The radio station wants all of the shows uploaded to the server to be MP3 320 kbps, so I bounce each show down no sweat. I always check the file before I upload and lately the MP3 file is a lot hotter than the original mix. Not just a few stray clips but a good bit hotter, I'm just not sure what's going on. I usually check it with Quicktime Player and the first few shows were fine. Now just to see if it was a Quicktime thing I loaded it back in to Logic and the files still meters hot, iTunes, Soundtrack Pro all the same. It's really the main monitor output on the Apollo, as it doesn't seem to show up in Logics main meter, but it was 4a this morning when I ran that test and I do plan to go back and futz with things a bit more. The music tracks are all music that has been mastered, and granted I'm all over the place, new, old, etc -it's been pretty easy so far to keep the levels under control. A few of the modern tunes I have played I have had to create a separate tack and take it down a dB or so but that has been about. I'm really picky in terms of my levels so I am always aware, I'm just not getting out what I put in, which I have never experienced mixing in Logic since version 7. I do have LPX so I guess I could give that a go. My laptop is old so I was trying to stick with LP9 in case I had to produce a show away from the studio. 

I normally record music / my band, etc and have a lot of control on the way in. I have never really worked with MP3 320 kbps format so is there something I should be looking for here? I'm hoping its something simple that perhaps I have done. 

Thanks in advance for the Help! 

~delton