Hi, Your symptoms are suggestive of uterine tightening, probably triggered by your diarrhea. It is common for mild contractions, the Braxton-Hicks to occur, sometimes they may be too many and too long, when you need rest and medication. When you have a consistent low back pain at this stage of pregnancy, it indicates that labor may be setting in, which is not wanted. You should also get your blood pressure checked as you have headache and vision changes. I suggest that you immediately see your obstetrician for further management. You can in the meanwhile take rest, relaxation in the form of some meditation and avoid exertion. Wish you a safe delivery.