My F21 gf is sad because I'm M23 sad :(

I've been with my gf for 7 months. We're both college students and she comes over to my house often since we're very busy with class work and I don't have a laptop, only desktop.

I live with my dad and sister and my dad doesn't have any issue with her being here often. Before valentines day she was here almost everyday and all we do is sit in my room and do our own work. We sometimes go out for dinner or sit at the TV room for lunch and sometimes we have dinner with my sister and dad.

My sister is only sometimes home. On valentines day, my gf was home and my sister came home and said to me, without any warning, 'does she have a home?' and said she felt uncomfortable that my gf was here everyday in her own home. I just said sure.

My gf didn't hear this. Ever since that day I was a little sad sometimes and mopey I guess and couldn't help it because my sister began ignoring me. She was my best friend too.

I didn't want anything to change, since they still say hi to eachother so I never told my gf about this. Since valentines she's also been coming over less due to meetings etc and has only been here twice a week.

Today my gf is over and my sister ate dinner in like 10 minutes and went straight to her room and came back out once the two of us were in our room. I got sad again when I realised that and my gf noticed I got sad sometimes since then. I would talk to her less and just look sad. I didn't want to tell her because I didn't want anything to change, she's not coming over often since then anyway.

Today my gf said she's a little sad because I won't tell her why I'm sad and I don't want her to be sad and I think I should tell her but I don't know how :(

I really like her but when I'm sad, I'm just sad :(