More than likely, it's just caused from A) not training for a while and B) lack of activation work before your warm ups sets.

A lot of people have problems with glute activation. So even though you're doing a couple of warm up sets, you can still get a little tweak in the glutes. Dynamic work helps a lot (active movements, like leg swings, hip/glute bridges, high knee walks....). It only takes about 5 minutes before you start your warm up sets. They pretty much 'awaken' the muscle and prepare it for the coming work.

Now when you say you're doing some stretching, you're not doing static stretching are you? As in, you're not holding a stretch until it feels 'looser'? If so, stop. Static stretching should be done after a w/o, not before.

Also, being that you were only feeling it in one side, you may be slightly out of alignment. And being that you're a wrestler, I wouldn't be surprised if you were. Perhaps hitting up your local chiro for a quick adjustment would be a good thing to do, just in case. A lot of places will do a basic adjustment for 30-40 bucks. And trust me, it's good to get that done every once in a while just in case. Otherwise, if something IS a little off, and you let it go for too long, it can lead to worse conditions.

Not saying that you ARE out of line, but I think it's a good investment to do once every 6 months or so just to be sure.

The one sided-ness could also just be due to a slight twist in your form. Keep a tight eye on your form since you're just getting back into things.