Have you calibrated your computer screen?
Yes, it has been.
In addition to the above, if you shoot in Raw, the preview in camera is jpeg and the one in LR would be Raw hence the difference.
I presume you ahve checked the camera screen brightness? (presume you can change this in the camera menu) everyone banging on about your home screen.. if camera screen set to bright then your getting a false view and pics really are darker..
Check the adjustment of the brightness of your camera screen. There will be a menu item to do this. If your screen is set too bright and your exposure is too low then LR will reflect the under exposure.
Ah Kipax beat me to it!

Is this a high camera screen issue and lower monitor brightness?
There is one way to ensure you don't take overexposed photographs and that is to work by your histogram (rgb one even with accurate white balance.
do you mean calibarting the actual laptop I use? Sorry, I'm just a beginner. My external big screeen has been calibrated but not the laptop.
Definitely worth checking if that one you're shooting raw, the preview generated will more than likely have the currently selected JPEG picture style applied to it, which may include a fair degree of brightening and boosting of shadows which are then absent when you import the raw file.
+1If that bothers you then make sure to switch your camera to neutral profile.