IIN for your GF to sometimes feel lonely while she's dating you?

No, you aren't silly - that seems like a very legitimate thing to question. You might, but I doubt you have a lot to worry about. (:

Maybe she is referring to having friend-friends which she is lonely for. Because, there's a different between the companionship of a boyfriend/girlfriend to that of a friend.

For instance, if you two were stranded on a deserted island and you only had each other for company. You are in love with each other but, naturally, it would be healthier if you had a conversation with a different person now and then---and not just with some stranger, but a friend whom you like.

The love for friends is different. You need them about as much as you need your girlfriend/boyfriend, but the love is a different sort of love. I think if she were lonely because she's unhappy for you, she wouldn't say something like this to you; unless she was accusing you directly of making her feel lonely - but then that's just a whole other can of worms, and everything I've written would not apply.