HCall - first of all, welcome to a WONDERFUL team, and great job on your 5K!
Second, I also ran my first 5K after only running on a treadmill, and I will tell you, you DO run faster outside than you do on the TM. AND you tend to run faster in races than you do on your own because of the race environment and adrenaline. I know even now when I feel like I'm running very slow outside it's usually a quicker pace than my regular TM pace. So that could definitely have been part of the cause of your side stitch.
Just keep running, outside when you can, and you'll get used to the differences.
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