diarrhea during pregnancy?

29 weeks pregnant and this week alone had diarrhea twice. not serious as I only went to toilet for less than 4 times. Stomach churned with grrr grrr sound like gas, no vomiting, no fever or chills. Went to see the doctor and she didn't seem too concerned, and asked me to go to the hospital in case of pre-term labour if the stomach ache gets worse (ok this scares me). Anyone has the same problem?