Feinstein, 89, said in a statement that the diagnoses came during the February recess, which was one week after she announced that she did not plan to seek re-election.
"Shingles is a viral infection caused by the varicella zoster virus, and it's the same virus that causes chicken pox," said Dr. Michael Stacey, chief medical officer of Lifelong Medical Care.
Stacey says while most people have chicken pox when they are young, the virus can remain dormant in the nerves and re-emerge decades later. One signature symptom is an extremely painful rash.
"A lot of people describe it as like a burning, stabbing pain," said Stacey, "It might last for a week to 10 days. Some people will have long-term nerve pain after getting shingles that can last longer."
Stacey says shingles is generally not life-threatening, and the risk increases as people get older.
"So once we get to age 50, the risk for shingles goes up a lot, and so the CDC recommends that we get the shingles vaccine at age 50," said Stacey.
Feinstein's recovery has caused her to miss about a dozen votes and several committee meetings.
"If this had happened a year ago, it would have been pretty significant because we had a split Senate and every vote counts," said Christine Trost, executive director of UC Berkeley's Institute of Governmental Studies.
Trost says this session, however, Democrats have a little more leeway, having picked up an extra seat that gives them a 51-49 majority in the Senate. Vice-President Kamala Harris also was able to act as a tie-breaker during some of this week's close votes, including one to confirm a federal judge.
Feinstein said in her statement that she plans to return to work later in March. Trost says if Feinstein's recovery drags on, it could lead to some tight Senate votes on contentious issues.
"The looming debt ceiling crisis coming up, that's going to be a very important vote," said Trost.
In the short term, however, Trost says she doesn't see any major disruption in representation of Californian's concerns.
"They have staff who ensure that constituent concerns are being addressed. They're extremely well-trained people, they've been there for decades," said Trost.
Feinstein is not the only senator to miss votes this week. Democratic Sen. John Fetterman of Pennsylvania was out due to medical issues. Sen. Jeff Merkley was out following the death of his mother. Republican Sen. Mike Crapo also was absent.
Jana Katsuyama is a reporter for KTVU. Email Jana at firstname.lastname@example.org and follow her on Twitter @JanaKTVU or Facebook @NewsJana or ktvu.com.