Sorry to hear of your problems, but if you have candida, you should probably start eating some live yogurt or taking some acidophilis caps. These put lactobacilis back into your gut. This is the good bacteria that naturally colonates the gut and this will help to put it back and could help.
Next, make sure that your eating a good all round diet with plenty of fruit and veg, fish, chicken, pasta and all those sorts.
Make sure that you drink plenty of fluids, up to 2 to 3 litres a day if you can. It is also a good idea to add in prune juice as your doc suggested. You need to drink it regularly though. Try eating cereal for breakfast also.
Try taking Metamucil or Phyllum husk to put some more fiber in your diet. If you need to soften your stools you can take docusate sodium. Metamucil is water soluable and you can take it once or twice a day depending on your needs.
If you still can't go, then try a couple of glycerine suppositories. If you still can't go after all that little lot, the try a baking soda enema. This is more natural and you can make it up yourself.
If you had tests that have come back negative, then it sounds as if you may have IBS and yes, if you are constipated, then you can have low back pain as well as the severe pain in your belly that you describe. Does your belly ever get bloated after eating?
Are you under any extra stress at present that could be causing this?
Anyway, hope this helps and post back.