vomiting and diarrhea sign of pre-term labor?

I woke up last night and vomited, then had diarrhea for about an hour. I thought I had food poising but no one else in my family got sick. We all ate the same things for the past several days. I'm 30 weeks pregnant and I heard this happens to women before they go into labor. Also, for the past 4 days I can hardly walk or turn over without excruciating pain in my lower back, butt, and thighs. Have any of you experienced this?