Several things can contribute to this, like I said hormones play a big role. But other things like medications can really mess with your skin as everything we put into our bodies come out thru our skin and hair. Why do you think they can find out if you have been using drugs thru a hair sample??
So if you have had a recent surgery that as well can mess with your skin. The medications and the anesthesia can play havoc too. Other things like pregnancy and menopause which can cause some extreme fluctuations in your skin can cause your tan to come out lighter, darker or patchy sometimes.
I know this all sounds bad, but I always want my clients to be in the know, if you just had a surgery or are in your first trimester then wait a week or two then make you appointment. Simple fix and a wonderful tan will ensue!