I have a five year old female pit-bull and she is having pain in her back legs. I'm not sure what it is though, she cries whenever she gets up from laying down or if she tries to get up on the bed or couch, it just started yesterday, also once she is up it takes her a long time to walk anywhere what could be causing her all this pain?

Could she have hurt herself and you didn't notice?
How old is she?

If it is an arthritis type thing and she is an older dog she may need some extra omega fatty acid and you can also get some Hylaronic acid for the vitamin aisle at Wal Mart.

It is best to first take her to the vet and find out the actual problem

Possible causes:
Pulled muscle
Damaged ligament
Arthritis
Hip dydplaysia
Cruchet ligament

There are too many things that could be wrong to give a diagnosis.

She needs to be seen to by a vet to determin the exact cause of the pain.
The things listed above are all treatable and manageble.