Yesterday was the outdoor one, right?
There is a little bit of difference on outdoors vs treadmill. When you are at home on a treadmill you are safe, you have everything with you. No excitement, just running. Your body lean is a little less - you are more upright, and your calves work a little harder. No wind resistance, but also no wind cooling.
But mentally there is less stress because there are no “unknowns”.
Outside there is weather, a lot of unknowns - do you have enough stuff with you?!?, will I be able to make sure I get back home safe, uneven terrain, wind in your face, etc, etc.
So it is possible the mental stresses of being outside hit you a little bit too. I don’t know for sure if that was it, but just something to consider.
You might spike in your race because of the mental stress of it too. Don’t worry, we will get there.
Here, this is a useful summary of hormones that can do this and how they work differently. This is not a medical description, just an overly-simplified explanation.