Williams was expected to stay overnight at the hospital. Angels outfielder Torii Hunter said Williams "passed out" in the clubhouse after leaving the 5-3 loss to the Giants in the fourth inning.
"I found out in the fifth inning when the trainers came down and told us that Jerome was on his way to the hospital," Hunter said. "It was a tough thing to hear. We didn't know what was going on. We knew he passed out in the locker room and had trouble breathing. At that point, all you could do was say a little prayer. Hopefully he's doing all right. I hope all it was is that he was a little amped up for the game."
Williams (6-5) gave up seven hits and four runs to the Giants in his third straight defeat and his second-shortest outing of the season. The journeyman pitcher improbably revitalized his career with Los Angeles late last season, but has yielded 16 runs in his last three starts after a strong start to the year.
Angels manager Mike Scioscia had few details on Williams' condition after the game, but said the team's medical staff was in contact with him.
"I think we feel much better right now because he's resting comfortably, and I think he's settled down a little bit," Scioscia said. "They're going to run some tests and see what they can find. I think he's feeling much better right now than he did when he left, but obviously, we're just taking it one step at a time. Hopefully it's not going to be anything serious."
Scioscia said Williams reported no discomfort or abnormal feelings during the game.
Williams is a former Giants first-round pick who spent parts of three seasons with San Francisco. He later pitched for the Cubs and Nationals before dropping out of the majors, failing to get back to the big leagues after signing with Minnesota, Oakland and the Dodgers.
He played in the independent Golden League before spending 2010 in Taiwan. He also pitched in Puerto Rico before joining another independent league early last year and eventually attracting the attention of the Angels, who signed him to a minor league deal exactly a year ago last Sunday.
He pitched well in Triple-A for the Angels before going 4-0 in a late-season call-up to the big leagues. He then beat out rookie Garrett Richards for the fifth spot in the Angels' rotation in spring training and got off to a strong 6-2 start to the season, stretching his winning streak at Angel Stadium to eight straight starts.
But Williams has lost three straight starts for the first time since May 2007 with Washington — and his rotation spot would be in jeopardy even without his health problem.
Angels ace Jered Weaver is returning from a stint on the disabled list Wednesday against San Francisco, and it's likely either Williams or Richards will lose his spot. Richards (2-0) has produced three straight outstanding outings since taking Weaver's turn, culminating in eight innings of scoreless four-hit ball against Arizona on Sunday.