I left my small pimple alone and the next day it popped by itself , is this bad ? what does this mean?

This is fine.: As long as you didn't squeeze it or pop it yourself, this is normal. The reason you shouldn't squeeze or pop it is so that you don't introduce infection. Pimples sometimes "pop" on their own, because of the fluid that has built up inside them releasing in the healing process.

Healthy: Normal progression of a pimple is to come to a head and then heal itself. The fact that it did not turn into a large abscess means that your body is healthy and mild infections heal well. Picking pimples will often cause the infection to go deeper and cause a larger scar. You did the right thing.

Healing: Acne pustules and comedones heal faster and better when the sebum/pus/blood drains out. It is traditional for parents and ignorant DRs to scare you about squeezing zits. Everybody squeezes zits. This helps healing and reduces scars. Acne is not your fault and scars are not your fault. If you have significant acne a dermatologist will help you.