If I sit down after a meal, eg eat my meal then just move to the sofa and sit there watching TV for two or three hours, my blood sugar will go higher than if I’m active after the meal. Sometimes I’ll need more insulin (called a correction dose) if it goes really high. Conversely, if I eat my meal, pick in the washing, put the bins out and/or take the dog for a 15 or 20 minute walk, my blood sugar will come down. I find it easier to control my blood sugar after a meal if I’m a little bit active. Some days I just go up and down the stairs putting things away, and even that helps.
It’s unusual for your blood sugar to go up unless you lie/sit after a meal - very unusual. It could be that lying/sitting makes you feel more relaxed whereas being active makes you anxious and stressed, so the stress is pushing your blood sugar up. It could also be that your insulin doses aren’t quite right yet. If you want, you could experiment by eating the same easily countable evening meal a few nights in a row and try various activities to see what happens to your blood sugar. If you go out for a brief walk, remember to take hypo treatments with you.
Generally, exercise will lower blood sugar (eg a walk or a swim). However, some people find very vigorous activity will raise it (eg sprinting). Don’t let the Type 1 stop you exercising. Build it up gradually and cautiously week after week
@StephenB