This situation is more common than you would expect. Several of my male patients have reported being able to maintain an erection throughout sex but not being able to reach orgasm. It would be a good idea to keep your medical doctor appointment to ensure that nothing is wrong medically. Here are a few questions for you to consider:
Are you able to masturbate and orgasm? How long does it ususally take for you to reach orgasm when you are by yourself?
Are you taking any new medications, supplements, or were you drinking alcohol or using any types of recereational drugs? (All of these can cause delayed or difficulty reaching orgasm)

From what you are describing, this issue ahs only been going on for a few days and that you were able to cum during all of the sexual encounters prior to Sunday. If this is the case, it seems that this may have been an isolated incident and does not appear that there is a medical explanation for what you are experiencing.

From a psychological/relationship point of view, you appear to be in a healthy sexual relationship and I am wondering why you would think you would lose your girlfriend over this incident. You mentioned that you pleased her sexually when you were not able to reach orgasm, which sounds like you really care for her. How did she react to your inabilty to cum? Did she make an issue of it or was this something that was personal/embarrassing/upsetting to you? Attempt to relax during your next sexual encounter with her--it is likely that if you focus on not reaching orgasm than you will in fact do just that. Enjoy the experience of being with your girlfriend; if nothing medically is an issue, it is likely you will return to your normal functioning.