Bush, who has a form of Parkinson's disease, has been hospitalized several times in recent years for respiratory problems.
He attended the funeral wearing a pair of knitted socks decorated in blue, red and yellow books - a tribute to his late wife's work promoting literacy. "He is responding well to treatments and appears to be recovering".
Sources close to the family say the 93 year old is now in critical condition and things were looking pretty grave initially.
After her death, Bush said of her: "We have faith she is in heaven and we know that life will go on, as she would have it".
The previous president was hospitalized final April for 2 weeks because of a "delicate case of pneumonia".
After a lovely ceremony Saturday, former first lady Barbara Bush has reached her final resting place at the George Bush Library and Museum in College Station, and visitors are already lining up to visit.
In 2015, Bush broke a bone in his neck after falling at his summer home in Maine.
The former president suffers from an infection that has become a blood sepsis.
Bush served as president from 1989 to 1993 and as vice chairman beneath Ronald Reagan from 1981 to 1989.