What would cause this sour smell?

I've been researching a little and I can't seem to find an appropriate answer for this. I have this awful smell in my house, almost like something is rotting (and dead). However, that can't be it because my partner can't smell a thing and thinks I'm crazy. I thought I might be crazy too, so I made a mental note to check out the smell after I got home from work. I didn't smell it at work, so it's not my imagination or something happening with me and when I got home, I was hit by it when I walked through the front door. Now, we are clean people and when I say that, I'm not just saying it. I have OCD and things must be clean and a certain way. Everything is dusted, swept, mopped, scrubbed and in it's place. We live in an older apartment building and I have read to check out the ducts and whatnot, however, it's not our place so all I could do was stick a digital camera down the vents and snap a photo but saw nothing out of the ordinary that would cause this stink.

Here's the kicker, this smell didn't start until it got warm and damp outside. It also seems to be mostly in the bedroom and just travels outside into the apartment. We have a small one bedroom and everything opens right into the other rooms so the entire apartment smells now. I have trouble sleeping at night because of this and I'm not sure what to do. My only guess is this is mold? If anyone has a clue, please fill me! Thanks so much in advance.