I have very little water pressure in bathtub only. Everything else in this house works fine. What could this be from?

When did this start and are both hot and cold equally low in pressure? Sorry to start with questions but sometimes we must before we can offer a fix.

Its gotten really bad in the last year

Oh and both hot and cold are equally low

There is not aerator on a tub so that's not going to be plugged. I'd open up the cartridge and clean it. While the cartridge is removed cover the opening with a towel and have someone else open and close the main quickly to blow out any debris.

Thanks so much!