The Mets were on alert yesterday when their SNY producer was put in isolation with a possible case of swine flu. Adding to the team's worries is the health of center fielder Carlos Beltran, who missed his third straight game on Tuesday night with an undisclosed stomach problem. Beltran's problem is believed to be a gastrointestinal virus, but nothing has been confirmed because the Mets are still awaiting test results. New York pitcher John Maine has been feeling ill as well. "These symptoms are not consistent at all with what Carlos Beltran and John Maine have at the present time, but our players are being informed that this has happened. Right now, no one on our team is feeling any of that type of symptoms," said assistant general manager John Ricco.