Pig's blood for a pig, for that is how Hargensen saw her.
Norman! What did you put in my tea?

That pretty much explains it perfectly. It's easier to get the blood, and it's a humiliating call back to her experience in the shower.
So many stories, so little time.

Pigs blood is easier to get. pigs urine really? you want that task? you must really hate Carrie White in order to go from swine to swine with a bucket under their hoohahs!
the basis was 'pigs blood for a pig' her being all mousy and hair drawn and dirty is where the pig comes from.
the blood also comes from what happened to carrie in the beginning and how she reacted to it. she didn't sit in class and wet herself, she had her first period and had no idea what it was.
Oh God. Fortune
vomits
on my eiderdown once more.