Don't feel bad, almost every guy has this problem at least once in life. It generally goes away with time. We can think our way into problems. So unless you've suddenly come down with a physiological problem, unlikely at your age unless you had some kind of injury or surgery, you're just overthinking it. You're basically killing your own buzz. I agree with the above, what is it about this particular woman that caused you to doubt yourself? Because it could be two completely different directions -- you like here in a way you've never liked a woman before and you're scared of losing that, or there's something about her your brain is warning you against. Figure that out and you've got your reason, most likely.
What you sound like you are dealing with is performance anxiety. Here's a link to a WebMD page on the topic:
If you've had this happen in the past on occasion, it might be that a more general anxiety sometimes affects you at a sexual moment. But if you have never had it happen before with any other partner (and you've had a lot of partners), there might be something about the relationship with this particular person that makes you feel more self-conscious or nervous, compared to before. And then, of course, worrying "will it happen again?" just makes you more self conscious and that doesn't help.
There are a lot of websites that talk about what (sexual) performance anxiety is and how to deal with it, I just posted the one above. but if you google the topic you'll find them.
Have seen a urologist and after some questions, he made an assessment of psychogenic ED and counselled me and prescribed Cialis for 1months... I just hope I get better soon. I feel better this time though occasionally worried. Thanks you all for your kind words