I use to get this as a teenager all the time, everytime i took a step i would get a pain going from my left hip down to my foot. I havent had that particular type of pain for ages, but my hips to pop out (eg if im in bed and i roll over, sometimes my leg doesnt follow). now i get more the pain down the back of my leg, from my lower back. ive put it down to siatica, where the siatic nerve gets 'pinched', this is common during pregnancy, with the extra pressure on your pelvis and hips, to compensate your body releases a hormone called relaxin, which helps your joints and ligaments become softer and more flexible to allow room for the growing bubs

a physio would be a good person to see, they would be able to give you exercises to help you support your body, and even some tools to help (eg a tummy belt).