I've tried kind of massaging it to see if she was constipated but no poo is coming out.
I have seen her go (RARELY) and I've tried feeding her pumpkin seeds for fiber but if anything it's getting worse.
Does anyone know what this is?
Ouch, looks like it could be an abscess or even an infected anal gland (common in dogs, don't know about rats). I'd look to take her to a vet as soon as you can. Until then, try to do warm compresses with a damp cloth.