Can someone explain how to properly use a "pain scale"?

Hello. I'm seeing a doctor next week, and I'm going to need to try and describe my pain on a scale of 1-10. I'm autistic, so I find using pain scales difficult.

In the past when I've been asked by health professionals to rate my pain on a scale of 1-10, I haven't been believed when I rated my pain as higher, because I often don't show my pain visibly on my face, even when it's very intense.

The pain I am currently experiencing is debilitating. I can't stand, walk for long periods of time, I can't sleep, and I'm experiencing full body spasms from the pain. All I really can do is lie in bed and cry. I guess I'm just not sure what to rate this.