If you've had a gun that works well and recoils the way you like with your 13# spring, I personally wouldn't change the poundage as that will change how the sights track. With as much experience as you have with this gun and load, that would be a downer to have to re-learn your gun. Definitely consider replacing the spring, though, as all springs wear out and they should be replaced periodically anyway.
My guess--as others have guessed--is that you got some brass that was fired through a 9 Major, and you've since won the lottery 8 times. Sucks, but it's that or pay for new brass. FWIW, if I'm going to a big match I'll make an effort to use new (or certainly only once fired) brass so something like this doesn't happen... but it still sometimes happens. At my last monthly match I had the exact same thing happen with a friggin' .22lr round.
FWIW also, one way to avoid 9mm Major left-overs and still use range pickup brass is to attend and get your brass from a steel match. Trust me, you'll have 9mm galore, and the chances of somebody shooting 9 major on steel is almost zero. There’s still a cgance youll get some as SC and USPSA shooters intermix, but I think the likelihood is significantly reduced.