A team source told ESPN NFL Insider Adam Schefter that Hayden's injury was related to a previous heart surgery. Raiders GM Reggie McKenzie said Hayden underwent the procedure late last week.
Coach Dennis Allen said Tuesday at the start of Oakland's second week of OTAs that Hayden first felt symptoms last Tuesday or Wednesday. Hayden has been released from the hospital, according to a team source.
Allen said he did not know if the latest injury was related to the torn blood vessel that nearly killed Hayden last November after a practice collision with a teammate at Houston.
"We can't rule that out, obviously, but right now I don't know exactly what the correlation is to it," Allen said. "But we don't anticipate it being an issue."
Hayden was rushed into surgery last year for a tear of the inferior vena cava, the large vein that carries blood from the lower half of the body to the heart, after the collision. Doctors had to cut through Hayden's sternum to save him. The injury is 95 percent fatal in the field, according to doctors, and is most commonly associated with high-speed motor vehicle accidents.
The cornerback overcame that injury to be taken 12th overall in April's draft. The team said doctors had cleared Hayden and he was at no further risk of injury.
Information from The Associated Press was used in this report.