If the bubbly urine doesn't happen all the time then it's usually not harmful and indicates a small amount of protein in your urine.
However if the bubbles in urine happen often or every time you urinate then that is an indicator of too much protein being in your urine which is not normal or good at all.
Bubbles in urine that happen often and indicate protein in your urine usually means that your kidneys are not functioning properly and allowing the protein to enter your urine instead of being filtered out by the kidneys.
Either way I would get a blood test done at the doctor to be sure everything is okay.
Bubbles in urine is not a normal thing either way so get checked out.