Post knee replacement and elderly

My mother- age 83 had knee replacement 2 years ago...(followed by 3 manipulations).
She has done pretty well after lots of rehab and time.

Recently she is experiencing what she describes as a rushing feeling (?blood) flowing inside her knee down to her foot. There is no precipitant that she can identify. She says it happens suddenly....feels very wierd and it rushes down her leg (internal)...She has also recently noticed that foot is getting cold very easily for no apparent reason.

Any thoughts please????