As per your symptoms, you could be having either bowel irritability or some excess prostaglandin
release. You should also exclude infection of the bowel. I advice you to consult your doctor and get treated as such diarrhea
may lead to preterm labor; moreover you should not be getting cramps at this stage of pregnancy
. You may need medication to control the frequency of stools and also the cramps. The burning sensation is probably due to sensitivity of the nerves that happens in pregnancy. Take care.