First off, I'm not going to even bother telling you why you should never buy from PetSmart. My first three girls were bought there before I knew really anything about cavies, and the fourth one was a surprise from my mom, same place unfortunately. I love my girls and I wouldn't trade them for anything in the world, but I do regret that I didn't adopt or rescue.
To me, she looks like she could be pregnant. You should start to notice her looking pear shaped. However, one my sows, Zoey did look pear shaped for the longest time and I was almost positive she was pregnant and was gaining weight. However, when I took her to the vet she had gained nothing and she wasn't pregnant.
Also, I hope you have another Guinea Pig of the same-sex; they are natural herding animals and are social. If you don't, you might want to look into adopting another sow or a neutered boar.
She should be having veggies twice a day, everyday, not some almost everyday. If you look under diet and nutrition you will find a thread of sample veggie menu's. I follow Jarbax's Veggie Menu. You will want to give variety, not just the same vegetables everyday, and create a good, equal balance.
What pellets are you feeding? The only real good ones are Kleenmama's (which can only be ordered online) and Oxbow Cavy Cuisine or Performance. (Performance for piggies under six months) Also, unless she is under six months (PetSmart sells normally young Guinea Pigs, so I assume) she should have 24/7 unlimited timothy hay. However, if she is under six months, then you are feeding her fine haywise as long as it is unlimited 24/7.