Just keep playing with it - the shutter speeds aren't all in order. All of the ones below 1/50 (1/60 for me) are not right beside 1/50. Hit your shutter button again, perhaps, and otherwise just keep pushing buttons. (lol, sorry I don't know off-hand - but I have experienced this, too).
If you have DRS (dynamic range stretch) on, the shutter speed will not go lower than 1/50 (60 fps if you've NTSC) even when you aren't recording. Though, this is only in 1080i50 or 720p50 recording modes. DRS is otherwise disabled.
So otherwise, there is a button behind the LCD panel called "Shutter". Press this button and it should toggle the shutter on/off. The shutter being off just means that the shutter is synchronized to your frame rate. Turn the shutter on, and you can cycle through the shutter speeds with the two speed select buttons.
The first shutter speed is the default "synchronized shutter speed" to your frame rate. Pressing the + button will get you to the lowest shutter speed, and you can cycle to faster shutter speeds by pressing +, and you'll eventually loop back to the synchronized shutter speed. Depending on the frame rate you choose, you will have different shutter speeds available.
In 30 and 60 fps modes, I have a shutter speed that goes as low as 15fps on the 150.
In 24p mode, the shutter speed only goes to 1/24.
in 60 fps modes, the shutter speed goes as high as 1/2000.
In 24p and 30p modes, the shutter speed only goes as high as 1/1000.