Could be baby's position or the fact your 36 weeks and carrying alot more weight causing lower back pain. All which are normal. Unless your getting timeable wave like pains associated with back labor I wouldn't worry. Put some icy hot or a heat pack on it and lie down.
If it's not consistent or timetable I wouldn't worry too much then. If it were I would definitely put a call in to your doc. If you're still worried, it also can't hurt to call. It's their job to answer your questions, you know.
Your whole lumbar shifts to accommodate the weight in the front, i've had it consistently throughout the pregnancy. Heating pad, and a pillow on my lumbar to support my back make me comfortable, but really... it's just part of the party.
Re: back pain
ETA: wording