Taz - a lot of those symptoms are identical to what my Mum had and she kept telling her GPs that something was wrong. They advised her about diet, said it was IBS, diverticulitis, told her to rest etc.
In the end she too, was in tears in the surgery and a GP finally agreed to do a rectal exam which took a couple of seconds before he realised she knew what was wrong with her. He immediately said he could feel a mass in her rectum.
Anyway, it was indeed bowel cancer and she had surgery and a colostomy which wasn't a load of fun but she coped. Almost 5 years later bowel cancer again occurred (but not spread apparently, this was a different "strain"). She had more surgery and now has very little bowel at all but is 87 and still going strong.
Because of all this I have had genetic counselling and they initially said I was at low-med risk of bowel cancer. When they saw all Mum's medical notes they have now said it's med-high risk because she had 2 separate bowel cancers. I've had diarrhoea for about a year and they never seem worried because I haven't lost weight and the diarrhoea is not all day.

Mum never lost any weight, neither did the girl in the story above.
Your life really is in their hands. I tell you what - when you're 60 you get a test to do at home for bowel cancer - just make sure you do that. I'm not old enough and when I asked for one they told me I couldn't, which seems daft as they cost pence!