Googling "bubbles in catheter tube" (both with and without quotes seems to work) produces sites which reference the term "pneumaturia." That seems to be the medical term for what you are describing.
The above-mentioned googling produces a number of articles. Sampling them seems to produce a consensus that what is happening to you is that air is somehow getting into your bladder and then being expelled through the catheter along with the urine, and this is what you are seeing, in the form of small bubbles.
The articles also list various causes for this, why air is getting into your bladder, some more serious than others, along with suggested remedies.
Perhaps the Wikipedia article is as good a place as any to start reading about
this:
/en.wikipedia.org/wiki/Pneumaturia The articles also suggest: see your doctor.
Good luck.