You could be carring more in the back. My son killed me, my daughter was ok and this one im carring in the back and my god i feel like im dying.
Could be your sciatic nerve, even though for most that doesn't come on till much later in the pregnancy. Next appt talk to the doc to see if there's something you can do to help..I'm 27 weeks my sciatic nerve is a day ruiner but I usually feel better if I tough it out and do a LIGHT walk and some back stretching. Everyone's different though so no need to over do yourself if you feel that's not working. Good luck and congrats on your pregnancy !