Suddenly diminished performance. Help!

I've been in a sexual relationship for a while, and until now the sex has been absolutely fantastic. I (male) have lasted at least ten minutes pretty much every time, and brought my partner to multiple orgasm fairly regularly. We would often remark, after sex, about how incredible it was. As I'm sure most of you know, feeling accomplished and satisfied sexually was an immensely rewarding part of my life.

The past three times, though (spread over five days), I've only lasted a minute or several. It's felt just like the first few times I had sex! I can feel myself reaching orgasm very quickly into the act, and I either pull out and cool off a little while (during kissing, of course) or just go for it. Either way, I feel dissatisfied, and my partner has not reached orgasm.

My partner is understanding, but I certainly am not pleasing her like I used to. Has anyone else experienced this? Any tips? I figure that I'll be back to my good ol' self after a couple more go-rounds, but I'm sure the self-doubt that has been instilled in me from these few episodes is not helping.