Why does she do that?

I have a 75 gal tank with 8 "feeder" gold fish in it. I have 5 adults, about 4 years old (they were only supposed to live about 2 weeks the person at the pet store told us); and 3 babies, one is about two years old and the other two are about a year old. My female sometimes goes in the corner and floats there just acting stand-offish. She only started doing this recently. She first started after I cleaned the tank. At first I thought she was in shock but she snapped out of it. When she started doing it again I thought she may have swim bladder disease. Now I'm just wondering if she is just simply moody. She has been better since I added another one of her babies to the tank. I can't help but to worry about her.
All my fish seem to have great personalities and always seem happy to see me. And it isn't just because I feed them. They are even happy when it isn't feeding time. I actually have a picture of all my fish lined up in front of the tank. They lined up to get their picture taken. I have to figure out how to post pictures on this place. I'm new here.

I changed about 50 gal. of water and cleaned out the filters. She has been happier since I added the other baby. And, no, she doesn't rub or scrape. Cearbhail, I checked your link and I'm considering doing the physical. It could be one of three things, impacted food, eggs or water temp. The temp has been dropping lately. Thank you both.