Hi, Welcome to Health care magic forum.
If you are getting the result, and getting cured ,you need not worry about the rest of the things.
If
penicillin vk, or any antibiotic is used for longer periods, and higher doses you may get the yeast infection etc.
If any antibiotic used in correct dose and for correct period it wlll not cause any harm. As you said if you take all the pills in one day you can have a yeast infection.
The pain in the flanx could be due to the
muscle pain, or chronic infection of the
ureter, or small stones in the ureter.
So always follow the instructions of your doctor you will be safe.
Wishing for a quick and complete recovery. Thank you.