My kitty is acting Lethargic!!! (I think)

I am a new member to this site, although I have been reading many discussions since I have gotten my first kitty. So here is my deal, if anyone has any advice at all! I don't want to panic, so hopefully it is no big deal. Anyways... My first kitty, Twink is about 6 months old. I just got another kitty (about 4 months old, now) a couple of months ago. They have seemed to get along just fine, and I don't think that is the issue but I am not sure. Twink has lately been really lethargic, and not responding to her favorite toy (an ice cube). I can't get her to respond to any of her rattling toys either. I am wondering if she is jealous of the newer kitty (On-X) because she is taking her toys? Twink has only been acting tired for 2 days now, and I don't know what goes on during the day when I am gone, but she used to be really playful at 2 a.m or so. She started shedding, which she had not before, but I also read from a discussion that it is normal for them to start shedding at 6 months old. She seems to be eating fine, and drinking and still gets really excited about her wet food at night. She normally has the "kitty" attitude that she want's attention when she wants it, but lately I have been able to pick her up and get her to stay, which is wierd. Any tips, advice, anything at all on what I should do or if I should worry. BTW... I have not taken either to the vet yet, but they are both going in a month to get spayed so I do not have a normal vet, yet.