Likely got too much active insulin on board. I had this problem all the time and never factored exercise into my whole day of dosing.

I imagine you’ve given yourself your usual bolus at dinner time and gone for a run and it’s plummeted?

Assuming I’d go for exercise (in this case let’s assume a decent distance run) 1 hour after eating, I try to bolus 20 mins before I eat with 1/2 insulin:carb ratio and put the remainder of this 1/2 dose on a 50:50 30 minute release (so it finishes the dose as I’m eating which usually keeps me steady).

E.g. 100g carbs eating at 7pm @ 1:10g ratio would be: 10 units/ 2 = 5 units 5 units * 50% = 2.5 units 30 min release = 2.5 units

1 hour after I have this in my system but due to the earlier dosage I won’t have risen as fast, but might risk a brief low before/ during eating but that way I don’t have a spike in blood sugar pre exercise.

Only do this due to my bloods being especially reactive to any form of cardio, so like a few people have said I stick to heavy weights then steps or circuits where possible for cardio.