Kidneys ache in the morning

Whenever I am on my back for over 8 hours (sleeping position does not seem to matter but I never sleep on my stomach) my kidneys ache for a while after I get up.  During the week my alarm goes off after 8 hours in bed and I never feel any discomfort.  If I try to sleep in on weekends, the ache wakes me up and takes a hour or two to subside and is gone for the rest of the day.  This has been going on for years.  My PCP has blown me off when I've brought it up.

The ache is localized to the kidney areas and does not radiate out to include my spine.  

I am taking cholesterol and bp medication and recently had an abnormal liver function test.

[Caryopteris]

Cholesterol meds are notorious for causing back pain.  You do need your liver function tests followed up.