Excal string stretch

[xbowbarry]

I'm really bummed as I write this. Took the Excal out for a late season hunt yesterday and really screwed up.
Had a buck come in and offer a 15 yard shot. Long story short I barely drew blood and we had some snow on the ground yesterday. Left him overnite and went back this morning and trailed him until there was nothing to follow. I think I just barely grazed him as there was hair and just a little blood on the bolt.
Came back home and shot the bow just to make sure it was human error. I was shocked as 2 bolts hit 8 inches low and 6 inches right. Had just shot the bow 4 weeks ago and it was dead on.
Checked my marks on the rail and my string was almost ½ inch off of the mark, it was fine 4 weeks ago. I know, I probably should have checked this before hunting with it BUT IT HAD ONLY BEEN 4 WEEKS SINCE I SHOT IT. My string is old but in great shape. Guess it’s time for my new "Boo" string.
My questions for you Excal guys are…
Is it common for a string to stretch this much in 4 weeks with no shooting involved?
Was I wrong for not checking my string with my rail mark?
Sick to my stomach,
xbowbarry