It could be a bad backache... It could be you have an irritated uterus... It could be your uterus stretching... It could be sciatic pain... It could be braxton hicks contractions... It could be a uti... It could be you are constipated... It could be nothing or it could be something...
33 weeks is a bit early for labor. Are the pains coming consistently?
Labor contractions have the following characteristics
they are regular
they follow a predictable pattern (such as every eight minutes)
they become progressively closer
they last progressively longer
they become progressively stronger
each contraction is felt starting in the lower back and then radiating around to the front or visa versa
a change in activity or position will not slow down or stop contractions
good luck at the doctor!