Due to your age I don't think it is anything to do with low testosterone or progesterone. I know you are healthy, but what about personally and emotionally, are there stresses in your life, have you and your gf become agitated with each other. Please let her know that this is NOT your fault and it makes you feel bad for her to get upset with you when you too want a good sex life back. When you start becoming aroused is it through your brain or by physical stimuli only? The reason why I ask is this, the brain is our largest sex organ. We become aroused by sight, taste, touch, smell, so IF something is bothering your mind and your emotional well being, you are less likely to become aroused. the usual only way is through manual stimulation. Just like being forced into it - as your body can not resist the touch, but your mind isn't in it. Think back to a week or so ago, was there an even or series of events that you might not have worried about then, but it is bothering your sub conscious?
The unfortunate thing is that your nerves and "freaking out" about it, is a stress in itself! You will be SO upset about it that this is all your mind can think about. What you need to know is this, ALL men and women have spikes and valleys of hormones, IF you can't pinpoint something to 4 days ago, then it will be harder to try and understand it. So since you can't think of something, you HAVE to take your mind off of it! Give yourself a day or two - just do regular stuff (NO SEX or masturbation) NOTHING sexual at all. You need to be able to get your mind off of your buddy, and focus on happier things. Even though it will be tough you need to be able to have sex without worry. The more worry the more flacid etc etc etc it is a viscious circle. Also your girlfriend NEEDS to be more understanding about this - I just answered a girl that is basically disgusted with the size of her boyfriends penis!!! There is FAR FAR too much concentration on sex and not enough of being in love and being happy. Sex is NOT you, it's what your body does when it is happy. So just become happy - take a break from it! And know that IF there is something physical going on, a urologist can help you with that! There are ALWAYS answers and ways out OK? Even though it's weird for me to ask - lol - let me know how you make out by next week! Just relax OK? This IS normal - not for you perhaps - but it happens ALL the time for both men and women!