Please help need advice

Any advice please.. My give year old has developed a fear of being sick or becoming sick.. She repeats the sentence "mummy I feel sick am I going to be sick " over and over and over again. No matter how I've tried to handle this its not working I've sat her down and told her she will be fine...tried ignoring..  Tried rewarding..  Tried everything she repeats constantly.  Teacher said she says nothing about this at school and is a bright happy little girl most of the time but this obsession is getting very very wearing.  Don't want to go to a doc coz feel may make it worse for her. She is not showing any other ocd tendencies however if she is beside someone who coughs or sees someone ill on TV it makes her worry even more. I don't know how long to leave it before seeking help... I'm so upset and worried for her wee mind and I'm trying so hard each time she adjs just to say no darling you won't be sick you are fine... Sometimes I loose my temper and shout at her but this doesn't even stop her.... Please any advice x x x. Very worried