My cat won't stop peeing and pooping everywhere, HELP?

She does use the litter box, but also, she pees and poops all over my living room.

I don't know why she's doing this, it's obviously not a problem with the litterbox, litter, etc since she's using it regularly.

A little background info on her, she's 10yrs old, declawed in front, we also have another cat who is almost 2yrs old but she has never had a problem with doing her business anywhere else besides the litterbox. My older cat just started doing this about a month and a half ago, I thought she might be marking but since it's feces too, I don't think that's the problem.

Can anyone help? Any advice?

Have you taken her in for a medical check-up?  That's the very first thing I would do to rule out any health issues.  THEN and only then would I treat this as a behavioral issue. 

But one thing that might also help is making sure you clean up all of the areas with a good enzymatic cleaner.  If you don't, then she will be drawn right back to those areas.  If she is doing it on a throw rug, simply machine washing in hot, soapy water isn't good enough.  You must put the enzymatic cleaner in the washer too. 

So, medical check-up, thorough cleaning, report back. 

Agree 100% with the vet visit since this is new behavior. She could have a UTI or other issue and still be using the litter box as well. I have seen and had it happen with one of our cats in foster.