Your red blood cell count could also be elevated from sleep apnea (stopping breathing while sleeping). This is sometimes amplified when taking AS. You may not even know this is happening. Your body compensates for the lowered oxygen levels by increasing the red cells (which carry oxygen). Its not really a big deal unless you start feeling tired during the day. When your off of AS, the apnea may subside, and your red blood cell count will lower. Not a big deal.
Other than that, it could be your diet.
You should get your blood checked a couple times a year just to let you know what is going on.