The 32-year-old American suffered a third-round loss to Frenchwoman Alize Cornet at Wimbledon and retired during a doubles match, appearing disorientated.
Speaking to reporters on Monday ahead of competing at the Bank of the West Classic, Williams said precautionary tests would be required later in the season.
"I was just really under the weather," the 17-time singles grand slam champion said.
"I still have to go through a series of tests. Not right away - I'm right in the middle of the season - but eventually.
"Just to make sure it wasn't connected to anything that runs in my family, that sort of thing."
Williams, who faces Czech Karolina Pliskova in the second round at Stanford, said doctors had ordered her to stay in bed after her retirement at the All England Club.
"I was really, really sick," he said.
"Literally the next three days I couldn't get out of bed. Usually when you lose in a tournament, you just leave. I literally stayed until the tournament ended because I was not allowed to leave.
"Doctors told me 'you've got to stay'."
Williams practised regularly during a break in Europe after recovering and said she was content with where her game was at, less than a month out from the US Open.
"I am feeling pretty good about my game," he said.
"But as you know, I am never feeling great about it."