Pre term labor signs? Or normal?

Last week for about four days I experienced diarrhea, period cramping for hours at a time, nausea, very bad heartburn, very dull and painful lower back ache, having to pee every half hour or so and it stopped for a couple days and doctor said it could have been back labor that stopped on its own? And now since last night it's started up again. I'm only 31 weeks. Should I expect a preterm baby or have others experienced this and found out it's normal?