yeah, with my first i didn't get anything like that but with my second and this current pregnancy i had the same problem and was/have been practically bedridden, the doc seems to think its because i didn't get my muscle tone back and am a lot weaker in the stomach and back area!
just take it easy and if it gets worse make an appointment to make sure all is ok!
i hope your feeling better soon! there's nothing worse then being in so much pain and having a toddler to run around after!