anyone else??

Is anyone else having really bad lower back ache? I had an appointment today and was told I was 3 cm dilated and 80% effaced at almost 37 weeks (Thursday). Dr told me I could try walking if I wanted to get things rolling which wouldn't take much since this is baby #4. Well this evening we went to the park to walk, while hubby played at the playground with our girls. I don't think I walked much, maybe about an hour and now I'm in so much pain. It hurts to move, especially walk. Has this happened to anyone else? I've never experienced this with my other pregnancies.