Re: kidney pain?
A CBC is a complete blood count. It tells you about your red and white blood cells, as well as other important blood levels significant to your immune system and general health.
A CMP is a complete metabolic panel, which delineates how well your body is processing the myriad of endogenous chemicals and other substances. For example, if it was determined that there was too much protein in your blood (or urine, which is another good test), it would suggest that your kidneys are not working well enough to filter it (excessive protein consumption, regardless of kidney health, is detrimental to your kidneys). Also, if it was determined that you had high levels of creatinine (not creatine), or keytones in your kidneys, that would suggest that your kidneys have been working overtime to process toxins, and keytosis (I might be spelling that incorrectly) might ensue. There are other significant results from a CMP, but the kidney health is my primary concern at the moment. A doctor can also determine if you haven't been eating much (or well) from the results of the test.
I think these are good blood tests to have done, to make sure your body isn't reacting badly to a fast, due to a current illness, or a possible unknown predisposition to some undesireable state of health.
Sebek