To address everyone's comments (and I thank you all for helping me):
I used to record vocals with an Auralex Mudguard. I stopped using it after I thought I was done recording vocals. I took the mudguard off. Then I began to notice some places where the vocals weren't quite what I wanted them to be, so I began redoing some vocals (this is about a year later I thought I was done with the vocals).
And that's when I started thinking something was off with the sound - too much mids. So I think the issue may very well be acoustics. At the moment I don't have a condenser mic (I just ordered one and should have it by the beginning of next week). Then I can test my theory.
Someone suggested that maybe a cold or flu might have triggered my hearing to not be right. I have been dealing with allergies, so that also might be a contributing factor.
I have been using 2 sets of monitors, so that's not an issue.
As for a spectrum analysis of a recorded signal, I wish I were that smart enough to know how to do that.
Thanks, everyone! I really appreciate all your comments. I will record vocals again when I get my new mic, and then I'll report back.
Greg