When you flush the toilet the shower water gets hot why?

Code allows your plumber to supply three fixtures using 1/2 tubing. In most bathrooms, you only have three fixtures (tub/toilet/lavatory). So most plumbers only run 1/2 pipe to supply the bathroom. Code allows this. Personally, I find this layout a little short-sighted.

Anyways - when someone flushes, the toilet requires water to refill. Which means your shower is now sharing that water supply with the toilet as the toilet refills. What I love best is when you re-adjust the shower valve...then the toilet quits filling...and you scald yourself. Lots of fun there.

Your problem is caused by poor Plumbing (my opinion) or if you are in an older home, galvanized pipes tend to corrode from the inside which only makes the condition worse.

If you like to do things yourself, carry over a 3/4 line and pull 1/2 take-offs for each fixture and the problem will be solved.

In a straight : When you shower you are using both hot and cold at the same time and when you flush the toilet you are causing the homes cold water to be guided more through the toilet line including the cold water that you are using from the shower line.

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If a few Words, water pressure and flow are inversely related. The more flow you have, the less the water pressure. Since the toilet and shower are connected to the same water supply line, when there is flow into the toilet, it takes away from the shower. Since cold water is used to supply the toilet, it takes away from the cold water flow to the shower. With the same amount of hot water flowing, the temperature of the water in the shower will get hotter.