If you are sitting fully on your bench, there is a risk of cutting off blood flow or the nerves to your legs causing numbness.
If you are sitting too low, the subconscious result is to hunch your shoulders, stressing your shoulders/neck/pectoral muscles.
There should be no pain. If at any time something becomes fatigued or painful, stop, get up, do some stretches, do some other activity for a while and rest your body.
Weaving is filled with repetitive motions. This is a ‘feel the pain, stop now’ type of activity.
Most ‘ordinary’ chairs are not suitable for weaving because most are raked towards the back of the chair. If you sit fully on the chair you will be in poor position/posture for weaving. If you don’t have a bench, build the height of the chair up and fill in that raked angle so that you are at a better height and position to weave.
While I am on a ‘let’s not injure ourselves’ rant, consider how you hold the shuttle. You might want to change your grip to reduce stress on the shoulder girdle/pectoral muscles.
Do you need a bench? Not really. I have an adjustable bench for the Leclerc which makes it easier when students come so that the height can be adjusted for them. For the other two looms in my studio, I have sturdy stools. I know people who have adjustable music stools they find suitable, especially if they have more than one loom. Piano benches can be adjustable as are drummer’s stools.
Be aware of your body. Let it rest when it gets tired. Do something else that uses a different set of muscles. Get up and walk around. Consult a physical therapist for exercises to stretch those muscles that get overused.
No pain.