Im 17 weeks pregnant and did an upward dog stretch yesterday because i was having lower back pain. my abs feel really stretched now. could i have hurt the placenta or baby?

Highly unlikely: The fetus is well protected by the amniotic fluid in the uterus , the wall of the uterus, and your abdomen and thus will take a lot of trauma to affect it. You should be more cautious on the exercise you are doing however since your center of gravity along with your body contour has changed. If the activity feels uncomfortable, don't do it.

Unlikely: It is best to ask your gynecologist. Muscles in pregnancy are more stretchable due to relaxin hormone. The back is most likely muscle.