Secret cuck

I came home unexpectedly early one day to my live in gf if five years to find her in quite an odd mood. She seemed quite annoyed I was back early for no apparent reason. As I'd come in the front door, I'd heard the back gate close. I asked her and she said yeah me and Chelle (her friend) were having a brew and a chat but she left when we heard you were back.
I asked why as Michelle and I got on well as far as I knew.
"Coz she thinks you're a **!"
My irritated gf said.
I was pretty shocked and had no idea what I'd done.
My gf said at least now she could go shower which is what she was about to do when Michelle had called round.
I sat in the living room puzzling over this. I knew something was amiss but I presumed it was something to do with them and maybe my gf telling Michelle things about me. I'm not perfect and I could be a ** sometimes like anyone.
Anyway, I heard her come out the bathroom so I went up for a **. Now, what I expected was to be nudging her clothes out the way with my feet as she always left them in a like as she'd took them off it was one of those habits you know about someone you live with.
But the floor was clear. Weird. Very weird.
I checked the hamper and there were her clothes. I took her top out and her bra then her leggings which had her knickers in.
I could smell ** strongly. The knickers were soaking wet. As I unfolded then to see the inside of the ** I saw that it wasn't just her ** juice either. I sniffed. **. No doubt.
All of a sudden several things I hadn't thought much if before all hit me and I realized this had been going on for months.
Apart from the **, I could smell her ** and it was stronger than I had smelled it for a long time. It smelled like it did when we first got together. I presumed it just changed as she got older. I immediately understood that she just wasn't as ** for me anymore.
It's hard to explain how suddenly all this hit me at once. I started to shake my adrenaline was pumping. Without even thinking...I shoved her dirty knickers in my mouth and instantly got hard, I pulled my ** out and wanked hard and fast while I could feel and taste some guys ** on my tongue. I came within seconds. As soon as I did, I spat her knickers out in disgust and felt sick. I flushed the toilet and ran the tap to rinse my mouth out. I carefully put her knickers back in her leggings and placed all her clothes back as best as I could the way they were.

I've never been into cuckolding or anything before or since. To this day I have no idea why I reacted that way.

I never told her. That was two years ago.

We are still together.