Hello there
It is clear that you obviously care a lot about your girlfriend

. Just taking the step of coming here and learning shows that.
Mainly someone with lupus needs what anyone in any relationship needs - love, trust and understanding.
As lazylegs said people with lupus will often try and hide how bad they are feeling or they will withdraw and be quite silent at times. Often we feel guilty about spoiling other people's fun and often we need to withdraw and just look after ourselves. I find other people's demands nearly impossible to deal with when I'm feeling bad.
If your girlfriend knows that you are there, without you being pushy and in her face, that will mean a lot. Doing practical things for her spontaneously and naturally as if that were totally normal can also help a lot. For example when I am feeling bad my husband can see that immediately and he will automatically do the things that need to be done in the house and also send the kids off on some activity so that I can be alone and just rest. No-one else would even notice he's doing it. He certainly dosn't make an issue of it.
Another thing that is very important to me is still being able to feel that I am truly part of everything and having a laugh. I don't want to be treated as an invalid or be defined through my illness. When I'm not feeling up to going out I appreciate being able to stay in but not necessarily being cut off from the world. Instead of going out to a dance or an evening, friends often come round for a more relaxed evening at home.
Lastly those soft hugs and marks of attention that lazylegs and elaine mentioned are very important.
Katharine