I've had many sigmoidoscopies. If it's the rigid one it is an oblong metal object about
15cm. The GI puts gel onto it to insert it easily. It's uncomfortable and then a bit sore when he turns the corner to see a little further. The moment I even hint at pain he takes it out. It's over before you know it and is bearable. The flexible sigmoidoscopy is a longer (about
30cm) plastic looking thin pipe that is inserted once you've had an enema to flush everthing out. It takes a bit longer and a lot of air is pumped into you which I found quite uncomfortable but also is bearable. You're probably having the first one. I usually have that, only had the flexible one once.
Good luck - it will be fine!