My partner & i had intercourse 3wks ago. ever since the day after sex, i have moderate lower back & hip pain. i just found out yesterday i am pregnant! could this be ectopic pregnancy due to the pain or can pregnancy pain start from conception?

Not start that early: The more common causes of lower back pain include: back muscle strain, back ligament sprain, fractured vertebra (spondylolysis), slipped vertebra (spondylolisthesis), herniated disc, spinal stenosis, spinal arthritis, urine infection, kidney infection or stone; tumor in the spine, kidney, or elsewhere; fibromyalgia, etc... With such a long list of possibilities, one's doctor can examine.

More likely: the timing fits with an ovulatory cyst, ectopics usually become symptomatic around 4 weeks post conception. IF pain continues or worsens see your doctor, or urgent care/ER.