Bad egg smell in kitchen?

For the past week there has been an egg smell in the kitchen that has gradually got worse and can now smell it all downstairs and assumed it was coming from the kitchen sink. Its not blocked, the external drain is clear and fine. I've been flushing the sink with boiling water, bleach and it hasn't helped. It smells worse under the sink and in the adjacent cupboards. Could it be something simple that i am missing or a bit more serious? I am a council tenant and don't want to get fobbed off and have this drag on, the smell is making me feel physically sick now. Thanks in advance.