Being that the temp was 100 degrees, i think you may have some heat exhaustion. This can make you have diarrhea. The stiff neck and weak legs can be related to the shoveling of the mud. You know how you feel when you start to do some heavy working out after years of a sedintary life style? This is sort of the same thing, we use muscles we forgot we had, but soon remember after some serious lifting, pushing or pulling over a period of time.
If you decide to do this again you must take several breaks, and always drink a lot of water, not soda or coffee. It may take you a little longer to get the job done, but at least you won't feel sick and exhausted afterwards. When you are out in the sun doing heavy work, you should wet a towel and keep it on your head with the help of a base ball cap. It may look stupid, but who cares, at least you will be cool.
You should, if not already, started to feel better. Take something for the diarreah and drink a lot of water to keep yourself from dehydrating due to the constant poopin. If you feel worse and continue to have loose stool, a lot, you should see your doctor. What about mosquito's? Was the water just stagnating there when you began the job? Do you think you may have been bitten by a tick?