Caroline, take her to the vet. I don't want to alarm you (and Chelsea's not old) but kidney problems are common in older cats, and with our Rover, the first sign of problems we got was that he was throwing up all the time. Every time he ate, he vomited - the bile in his stomach from the faulty kidney made him sick. However - early diagnosis these days is brilliant, they do a blood test, and if it IS that, a tasty tablet a day can keep Chelsea going for years. Rover's 15, he was diagnosed at 11 and is still going strong.
It might help if you vary her diet too - all Iams may not be right for her, we use good quality food like Senior Felix.
Do get her to the vet a.s.a.p. if only to exclude kidney problems - the cat will be losing weight and condition if she's not keeping anything in.
...and I do agree with FGT, chasing her when she's trying to throw up is very stressful - just keep lots of kitchen roll and damp flannels handy.