Things that can change the colour, texture and size of your stool can be:
Foods (obvious one)
Food colourings, often found in sweets and candy and many drinks.
Drinks, both soft and alcoholic
Medications (a HUGE array of over the counter and prescribed medications can all effect the colour, size and texture of your stool, if you are taking any medications, check the side effects and see your doctor if you suspect this may be the cause).
Diet pills for losing weight (these are really well known for causing stinking multi coloured stools)
Rarer causes can be polyps (benign growths on the wall of the colon which may turn to cancer over years and are best out, diverticular disease, which causes small outpouching of the colon wall, which can become inflamed when irritated by stool getting trapped in them, causing diverticulitis. Some hormonal and endocrine imbalances can effect how your body absorbs fats meaning a lighter, foul smelling stool containing lots of fat, gallbladder stones (very detectable by ultrasound). All these symptoms need discussion with your doctor.
Blood (blood in stools can present as bright red, usually with an anal cause such as piles, fissure or abscess, or darker to black, meaning bleeding higher up in the digestive tract such as a stomach ulcer which can be treated and can contain a bug called H Pylori, which will need antibiotics, diverticular disease, or occult (hidden) which can only be detected with a FOB (faecal occult blood) test. ALL rectal bleeding MUST be checked by a doctor as one rare but possible cause is bowel cancer or polyps, small growths on the colon wall which may turn to cancer over a long time and will be simply removed during a colonoscopy, so if you notice signs of blood in your stool, get along to a doctor straight away).
All changes in your stool that you are not happy with should really be discussed with a doctor. Take a photo of your stool and go and see your doctor, they have seen, smelled and even touched more poos than you have had hot meals in your life.