Diane Feinstein has now reportedly been admitted to a hospital in San Francisco for treatment of Shingles
Mrs Feinstein, 89, was diagnosed with the virus in February.
Shingles is frequently seen in elderly people who have had chicken pox. The rash may remain and produce nerve discomfort.
Ms. Feinstein said last month that she would not seek re-election.
According to medical professionals, shingles is not usually fatal.
Mrs Feinstein has been in California since late February, and her spokesperson informed the San Francisco Chronicle that she had missed a dozen votes and two committee sessions since her sickness began.
Ms. Feinstein, who was first elected in 1992, said last month that she will not run for re-election in 2024.
So far, three California House members have declared their intentions to replace her.
She has served in various public offices, including as mayor of San Francisco and as a United States senator representing California.
Feinstein began her political career in the 1960s when she became involved in local politics in San Francisco.
In 1970, she was elected to the San Francisco Board of Supervisors, where she served for nine years.
In 1978, Feinstein became the first woman to serve as the mayor of San Francisco, a position she held for ten years.
Feinstein has held various leadership positions in the Senate, including as the chair of the Senate Intelligence Committee and as the ranking member of the Judiciary Committee.
She has also been a key player in several high-profile legislative battles, including the impeachment trial of former President Donald Trump.
Feinstein is now the second Democratic Senator to be hospitalized, following Pennsylvania Senator John Fetterman who was checked himself into a hospital outside Washington D.C. due to depression.