Why do I feel weak in the knees and generally unstable while pregnant?

Relaxin. Relaxin is a hormone that is released in pregnancy. It relaxes the connective tissue in your joints. This makes them looser and makes you more unstable when you walk or exercise. Relaxin is the reason pregnant women waddle. It is also the reason pregnant women are more likely to dislocate their joints. Take things slowly and use extra support when pregnant.
Blood pressure. The usual cuprit is low blood pressure. Make sure you are staying hydrated and, when getting up from bed or seated position, move slowly. If this persists, definitely call your ob.