Musty , burning smell?

Hey guys ! I have a resmed 11 auto sense and I’ve been using it for about 3 months now , so over the past couple weeks I’ve been noticing sometimes when I wake up there’s a stale , burning smell in my mask , but my water tank still has water in it and everything , any idea what it might be ? I’m not sure if it’s something I’m doing or anything, I have examined it and it looks just fine