Pronoun usage and pronunciation varies greatly between formal and informal French. Much of what you learn at school is formal and doesn't reflect how French is actually spoken. If you want to sound more French in informal situations—not to mention understand what you hear—you need to be aware of these pronoun differences.
Inversion with the first person singular je is a little trickier than with other subject pronouns. It's also very formal and therefore rare, so one of those grammar concepts you need to recognize but not necessarily use.