It sounds like you have a trapped nerve. Especially if you are getting numbness. As you are standing up when the numbness happens, it could be the baby pressing down on a nerve in your pelvis so when he/she has been pressing on it for more than two minutes, you get that numb sensation.
If its the begining of SPD (you'll know cos it will get worse rather than better), do exactly what Naturalmama said and go see an osteopath or Chiropractor for an adjustment, but one who knows about pre natal care. That should make things less painful...
Oh yer... and rest... its always a good excuse to rest and watch telly... 8)