At about 12 -15 wks in pregnancy the hormones in our bodies start to relax the muscles around that area to allow the baby to grow into places used by other bits (to put it simply!)
So everything can be moved around! When we are sick we lose the control and as those muscles have been weakened it is possible to wet yourself.
I had hypermesis with my DD - and from 14 wks I wet myself everytime I was sick. Its not nice and can be v embarrasing I know (I did it in my brothers toilet at a party - threw up down the loo & wet the floor!)
I went on to have a c section and my pelvic floors muscles have returned to normal the minute she was born. Don't worry about doing the exercises now - it won't help - its happening for a reason.