Richard Simmons went to the hospital Monday for "severe indigestion" ... according to his rep.

Richard had been battling "discomfort while eating" for several days, and finally decided to get treatment at a hospital on Monday.

His rep, Michael Catalano, told ABC News ... Richard was already doing much better and expects to make a full recovery.

Our sources say the "gastrointestinal issue" wasn't serious enough for an ambulance, and Richard's people drove him to the hospital.

This is the second time in the last year Richard's had to leave his home -- where he's been sheltered for years -- for hospital care. Last summer, he was at Cedars-Sinai for 3 days after his housekeeper said he'd become incoherent.

His people said that was a case of dehydration ... which seemed weird for several days of treatment.