Housing
What size is your tank? 15 gallons
What temperature is your tank? ~80
Does your tank have a filter? Yes
Does your tank have an air stone or other type of aeration? No
Is your tank heated? Yes
What tank mates? Two other female platys
Food
What type of food do you feed your fish? I mix it up between tropical flakes, crisps, and bloodworms (about twice a week)
How often do you feed your fish? Once a day, fast on fridays
Maintenance
How often do you perform a water change? Once a week
What percentage of the water do you change when you perform a water change? ~25%
What type of additives do you add to the water when you perform a water change? Prime Water Conditioner
Water Parameters:
Have you tested your water? If so, what are the following parameters?
Ammonia:0
Nitrite:0
Nitrate:10
pH:9.0 (Yeah, I know it's high, but that's what my aged tap water is. It's from a community well)
Hardness:N/A
Alkalinity:N/A
Symptoms and Treatment
How has your fish's appearance changed? It hasn't
How has your fish's behavior changed? I noticed that she swims a bit sideways every now and then. She also hides
When did you start noticing the symptoms? Today
Have you started treating your fish? If so, how? None
Does your fish have any history of being ill? N/A
How old is your fish (approximately)? 3-4 Months?
I just bought her two days ago, so I'm not sure about health history. For the most part she swims normally, but she occasionally turns to the side while she's swimming. Is this early SBD? She doesn't have any bloating. She's also been hiding more than the other girls (one I've had for awhile, the other one came home yesterday) but I didn't think anything of it, since she's new and adjusting to her new home. But now that I see her swimming a bit odd, I'm worried. Any suggestions?

I took a video of her swimming on my phone, I'll post it when I figure out how to send it to my computer

Thanks in advance!!