back pain

Hi, I am having a bit of a conundrum with a little guinea of mine and her elongating herself showing discomfort/pain. She was doing this when I first brought her home, aged 8 weeks. I thought she just liked stretching herself out - looking like she was chilling out like we all do on holidays...

So, I have a guinea pig named Caramel, and she is 5 and will be 6 in about 2 or 3 months. (Nobody knows her real birthday.) She seems to be going through pain lately, and I hate to see her like this. She has been sort of hopping, sort of limping on her back legs instead of walking on them, and...