The reason why these things happen is also the reason why the other things happen, to which you made reference. It is a cosmic dislocation making internal adjustments. These things, sneezing, stomach-ache, fever etc., are internal adjustments of the body to maintain its balance. It passes through certain peculiar painful adjustments for the purpose of maintaining a balance of health. No purpose is served.
Nature maintains a kind of balance where certain extremes take place, which maintain the balance.
When you eat too much, you will have a loose motion. Now, it is not a bad thing that is happening, though it is certainly not a happy thing. It is necessary for the maintenance of the health of the body. You have done some extreme thing by overeating. You have walked in the rain, which is an extreme thing that you have done, so you developed sneezing and fever.