You may have Irritable Bowel Syndrome. it can come on at any time of life. Have you been out of the UK and the local Montezuma's revenge, or are you stressed/anxious?
Go back to your GP and raise this with him. Keep a diet diary and a pooh chart for a couple of weeks. You may need further investigation such as blood tests, stool sampling and even a big black snake to have a look around but better to be safe rather than sorry.