I've experienced this first hand too. It's really frustrating and upsetting. (And I happen to have the same condition.)
But to make it worse...I can understand why people think that...and I'm not sure I wouldn't think that too if I didn't have personal experience of variable, invisible conditions.
So I decided to make a simplified version of my decision process in case it helps people understand why I can do something one time and not another.
This doesn't take into account my balance issues, which mean that things moving in my line of sight are likely to make me lose my balance.
Nor the consideration of whether to use crutches, wheelchair, knee braces, or try using just my legs - which is affected by which joints are playing up in which ways, coordination levels, fatigue levels, what I need to do tomorrow and more.
Nor that walking and keeping all my joints in line is like tightrope walking - and equally fragile.
Nor the 'lycra factor' - am I able to wear my lycra compression stuff (helps with my poor proprioception and reduces blood pooling and POTS symptoms) - and am I likely to manage to take it off again at night without injury.
Nor the fact that the situation may change half way.
Nor whether help is available if it goes pear shaped.
Nor a load of other things.
So yes, you might occasionally see me walk 5 metres looking absolutely marvellous, or get out of the car and walk to the boot and but you won't see the concentration and effort, or the after effects.
Similar decision making processes apply to all sorts of everyday activities.
And I suspect that people with a range of conditions causing chronic pain and fatigue go through variations of this decision process.
So if you see someone 'looking fine' one minute and 'looking disabled' the next, remember it doesn't mean they are 'faking it', it might mean that they have a complex condition that they are doing their best to manage.
[1/5/2016 Edited to add: Due to popular request, a more generic version of this image (see below) is now available as a poster from Stickman Communications ]