Random sewer smells in apartment

Hi, I live on the top floor of a 3 floor apartment block. Recently, at completely random times, foul sewer smells suddenly come from under the sink. This can happen whether I’ve recently used plumbing or not and has got really bad over the last few weeks. I’ve checked the u bend and it’s completely clean and not obviously leaking. And I have put drain cleaner down also. Water drains with no issue at all. It’s become near unbearable to live here. Any advice? I’m wondering if it’s an issue with my plumbing, or further down? And what could possibly cause it to happen so randomly? Thanks in advance and help!