Our 15 Year Old Female Cat (Emmy), Spayed Since Age

Our 15 year old female cat (Emmy), spayed since age 1, has been vomiting thick, clear liquid (and foamy at her mouth) repeatedly for about 2 hours. She's not vomiting nearly as often now. Her breathing is continuously labored. Since Emmy had been vomiting, she had two separate bowel movements, first one small piece, then finished about 12 minutes later. It appeared normal and healthy. She has no appetite, but she has only missed one meal. She normally would have eaten by about 6pm. She is not obese. Her shots are up to date; last shots given July 24, 2015. According to pet sitter, Emmy refused treats on Friday Oct 23rd. We were out of town for three nights, Oct 22 to 24. Pet sitter said Emmy was more used to her this time. We are rarely gone overnight, having been gone for three nights in August after not having been gone at all for 20 months. Emmy had a strange moment of inexplicable panic last night, less than 24 hours before vomiting began. My wife was in the bathroom at the time, which meant whatever slight, normal noises (as far as I could tell) coming from there would have emanated from directly behind where I was sitting. Emmy had looked straight at me for 10 to 15 seconds before panicking. No signs of fever - her head does not feel warm/hot. Still slightly labored breathing, not quite as bad. She cries out in distress, which she does as a habit before vomiting. Emmy is very close with me; I've known her since the day she was born. I don't know if she could be having a delayed reaction to the stress of my wife and I having been gone for three nights. Emmy did fine when we were gone for 3 nights in August. She only very rarely has hairballs. She doesn't shed nearly as much as our other cat, and they get along extremely well. The thick, clear, foamy liquid she's vomiting reminds me of what came from her mouth when she was nervous traveling to the vet for her vaccinations appointment in July. She has not vomited any food. She ate all of her morning meal at usual time, 6:15am. We moved her from my parents' house, the only home she had ever known, in March 2013, and this permanently separated her from her only remaining sibling at age 13. Four weeks later, she vomited food and couldn't hold anything down, even water. She had a high fever, and I nursed her back to health with small ice cubes and drops of water and eventually small pieces of meat. We thought she may have had a delayed reaction to the extreme stress, but we don't know how to explain the fever (in 2013). No fever this time. Could she be having a delayed reaction from stress?