Bed shaking begins-Late 2000/early 2011

So now that my GF lived with me and my adorable son was born, things sort of calmed down. Though they didn't stop completely. I noticed that my bed started to shake or vibrate at night. I now realize that this is usually a precursor to something happening, as it would normally accompany other events. My GF has felt it on many occasions and it has become part of the norm, with me anyways. It has also happened at other places than my original apartment, like my new apartment and hotels. It feels like someone is standing at the edge of the bed and lightly shaking it. Though a few times the shaking has gotten quite violent and scary.