I feel like my baby is gonna fall out when I stand up?

As soon as I get up from sitting down I feel pain to stand straight. i feel like i have to bend over to keep the pain from coming. i feel like my ovary area is being stretched painfully. i also feel like my baby is gonna just fall out and onto the floor but im only 18 weeks and this is my first baby. why do i have this pain. its kinda like my bladder is way too full. Even though i don't have to pee. I have a heavy feeling even toward my area too. HELP!