I bet it's your body adjusting to your stomach and/or boobs getting bigger and you carrying more weight out front. I've gained a lot in the last month and had to get a body pillow to sleep with at night b/c I was having a lot of back pain. The pillow helps keep me on my sides too. I worry about pre-term labor a lot too (not right now, but I worry about it happening in the next couple of months), so I definitely understand your concern, but I bet it is nothing to worry about.