erfly stroke swimming, back pain?

i also used to suffer from back pains - i'm no expert but make sure you put your head far down before your arms follow, (my coach told me that) and to get your body far out of the water. As you said your stroke is probably fine but after 4 or more lengths you start to tire and don't have the strength to get your body out the water... sorry i have no answer of easing back pain but i'm definate its normal.