I think she would be so touched if she could see your post. I think its great that you are being so supportive and understanding. I think it can be really hard to gauge how to respond to someone, especially when they are probably going through every emotion under the sun. Also with HMS, in my experience, each day seems to be different and I can be fine one day and then unable to walk with bad knees other days so I guess you have to tailor your support to the individual situation.
You know your girlfriend better than anyone and I'm sure you will learn in time to know how much obvious support to give some days and how maybe at other times just to be more subtle about it. I know I'm being really vague, I guess its just hard. Maybe talk to her and let her know your concerns about smothering her with attention etc, and let her know that she is free to tell you to back off if she likes
I just know that my ex was massively unsympathetic about my condition and it just makes me feel great that other people in couples are so brilliantly sensitive about it,