Horrible pain when I stand?

I am 36 weeks pregnant with twins. When I wake up first thing in the morning I have this really bad pain when I first stand up. The pain is in my abdomen and shoots downward. I always have to try and wobble to the bathroom every morning. Do you think this pain is from the pressure of the babies on my bladder? I also feel this pain sometimes throughout the day but its not as bad. Has anyone else ever experienced this during pregnancy? Thanks all.