How old is your dog?
What kind of dog is this?
Is she overweight?
OK thanks for the extra info!
Often when a dog has hard time going up stairs this is a sign of weakness in the rear because the rear legs take most of the weight on the way up.
Getting up can also be an issue if the rear legs have a problem.
Some possibilities include a knee ligament issue which you can read about here
A tick carried disease can also cause joint and muscle pain leading to a situation like this.
A hip problem or spinal issue can also be a pain source leading to rear leg weakness.
To help with her mobility you can sling a towel or a log carrier under her chest to help support some of her weight as she gets up or goes up the stairs.
As you don't know yet what is causing the problem there are no home meds that would be really safe to use.
I'd plan a vet visit for her. If she is getting steadily worse you may want to see an emergency vet for help.
This site may help you locate an emergency vet if you need one
But if you don't feel its that urgent you may want to plan to see your vet first thing on Monday.
If the sore area appears to be her tail this might be an extreme case of cold tail which you can read about here
which is seen in this breed.
Hope this helps you!
could it be anything more serious? as i said it seems to of just happened overnight and i'm very concerned about her.
It might be a disc problem. If she is getting worse you may want to see an emergency vet even if all they can do is give her a dog safe pain med.
If your dog might have gotten into anything toxic, has a rectal temp over 103.5F (normal is 100.5-102.5F), has gums or tongue that look white, blue, gray or yellow tinted where they are normally pink, has a slow color refill if you press on a pink area of gum or tongue, or is becoming lethargic and non responsive to you then you may want to get the dog in to see an emergency vet. The link above will help you find one if your vet isnt' available.