Williams was admitted after he experienced trouble breathing during the team's 5-3 loss to the San Francisco Giants. The right-hander lasted 3 1/3 innings, allowing four runs and seven hits.
At about 12:30 p.m. PT, Williams tweeted, "Wanted to let everyone know that I'm feeling better. Gonna get some rest so I can be back on that mound competing again."
He also thanked fans for their "thoughts and prayers."
According to the Angels, Williams will return to UC Irvine Medical Center Wednesday for further tests. No other medical details were released.
Angels right fielder Torii Hunter said the team was informed that Williams passed out in the clubhouse after leaving the game in the fourth inning.
Williams has a history of asthma.
Angels manager Mike Scioscia visited Williams in the hospital late Monday night. He said it's too early to decide whether the pitcher will be put on the 15-day disabled list. The Angels have rookie Garrett Richards, who is 2-0 with a 0.86 ERA in three starts, standing by to pitch this weekend against the Los Angeles Dodgers if they need him.
"Jerome felt much better once everything got calmed down a little bit and he continues to feel better today, but we have to evaluate exactly what the situation is," Scioscia said. "We'll just wait for some direction from our medical department."