Lonely kitten?

I have a 7 month old long haired domestic.

Until a month ago she lived with another cat (year and a half) and a puppy. Now she's alone and I find I'm away a lot of the time. I've had complaints from people in my building that she is crying during the day.

I've been taking her upstairs (where the other cat now lives) to spend time during the day and she is much better.

Is this something she will grow out of or do I need to get her another cat to keep her company?

I've been looking into being a foster cat owner, but do you think this will only solve the problem as long as I have another cat with her? And if I get one, would a kitten or a cat be better? She's not great with other cats, only the cat she grew up with. But I think she's young enough for it to be okay.

I think she's lonely, and I'm not sure what to do.

Fiona was around 2 years old when I got her and cried all the time. Even when we were home. We got ReeRee who was 3 months old and she stopped. We joke she just needed someone to boss around.

Why is she separated from the other cat? I got a pair of sisters because of this concern. Their tag team vet cost have put a huge dent into my bank account but I can't imagine having an only kitty. Especially since I'm away for the day and travel moderately.