I don't know if there's something wrong with my pet rat?

Okay so I have two female rats. They live together in their cage and I let them out to play all the time. I've had them 7 months. Ellie has ALWAYS been very social with humans, dogs, my rabbit, and my cat (yes I know). Every time you walk in the room, she runs to the door of the cage for me to let her out so she can explor my bedroom. She's soooo sweet. Elsa, the other rat, is sweet too, but not AS explorative and social with other species as Ellie, which I understand because different personalities. But the past few days, Ellie (the very social and playful one) has been extremely lethargic. She doesn't run to the door of the cage and even when I open the door, she hesitates to explore (which is VERRRYYY weird) and i don't know if it's just me thinking the worst but it almost seems like she's breathing heavier too. But the really weird thing is, Elsa's personality has changed to what Ellie's used to be. She's really playful. And when they are just chillin in their cage Elsa is always cuddling Ellie as if she comforting her? It's just so depressing to see Ellie go from so playful to randomly all sick looking.