Apparently during his physical at the combine an irregularity was detected with his heart:

Star Lotulelei sidelined by heart condition at combine - NFL.com
That could change after his NFL Scouting Combine physical. Lotulelei's agent, Bruce Tollner, told NFL.com's Akbar Gbaja-Biamila on Sunday that the player will not take part in combine drills in Indianapolis after an echocardiogram detected a heart condition.

Tollner said Lotulelei was asked to not participate in workouts until after being able to consult with a specialist. Lotulelei plans to see a specialist later this week and will continue to interview with NFL teams at the combine. Lotulelei anticipates taking part in every test at Utah's pro day March 20, Tollner said. According to ESPN's Chris Mortensen, who first reported the news, Lotulelei's left ventricle is pumping at 44 percent efficiency compared to the normal range of 55 percent to 70 percent.

For his stake I hope it turns out to not be serious.