RE: receiver battery
If you had 2 flights last weekend, and didn't charge the batteries since, I'd say you're right, You ran them down.
Nicads will lose power if they are just sitting. ALWAYS charge before flying.
BTW, many times you will see no noticeable change in control when nicads run down because once they run out of power, they don't lose it gradually, they drop like a stone. And so does your plane.