The Pomodoro Technique will feel a little different for different people depending on their abilities and tendencies, the type of work they do, the environment they do the work in, and the level of experience and skill they have with this type of work.
The important thing is to understand the theory behind the technique, be disciplined about reading the entire official Pomodoro Technique book, be disciplined and patient about trying it their way for a time, and be determined to progressively achieving the different objectives laid out in the book. It is totally normal to feel the 25 minute session is too short when you are first trying out the technique. Actually, according to the book, they did experiments where they had groups change the length of the pomodoro based on the observed effects. Generally, teams started off with 1 hour pomodoros, then increased it to 2 hours, then down to 45 minutes, then 30 minutes.
You said yourself that you were concentrating really well and it felt like only 5 minutes went by. It sounds to me that rather than hindering you, the technique enabled you to have great flow and concentration during those 25 minutes, which is part of the reason for using the technique. The Pomodoro Technique has many elements that enable this. First off, you specifically chose your current task from a list of all tasks you were considering, so you should not be suffering from lingering doubts that you didn't really consider your options first. You know that you only need to concentrate on it for 25 minutes, thus enabling you to fully commit to that task instead of being afraid that you should perhaps switch to a different task at any moment. Then you wound up the timer, mentally solidifying your promise to yourself and your determination to stay focused on that task during the 25 minutes. Only after doing that, do you hear the continual ticking which reminds you that you are in the middle of a pomodoro. Knowing that you will only do that activity for 25 minutes before stopping encourages you to get something concrete completed during each 25 minute session.
Taking a short break at the end of the 25 minutes has numerous advantages. The most obvious is that it gives you a break so you don't burn out during and stop being productive in future pomodoros. But it also gives your subconscious a chance to process what you just did. This subconscious processing enhances learning, facilitating both short and long term learning and problem solving. Often, I find that it is not until I am on my break and not thinking about or working on the problem, that my subconscious mind causes me to have a seemingly random epiphany about a better way to approach the problem. Remember, you are not supposed to do anything mentally complex during your break. Including the break, each pomodoro is normally about 30 minutes long. This is a time boxing technique. When you write out activities in your "activity log" and in your "to do today" sheet, you estimate how many pomodoros each activity will take. By keeping each pomodoro a short 30 minutes, this helps you break up your activities into smaller chunks, which improves estimation by forcing you to think more deeply about what is exactly involved. After each pomodoro is finished, you have an opportunity to change directions, or work on something that you realize is more important. Knowing that you can do this every 30 minutes, takes away the urge to consider doing it during the 30 minutes, enabling higher productivity during the 25 minutes. If the pomodoros become longer, it becomes more difficult to push off both external and internal interruptions. If you tell someone you can get back to them in 30 minutes, this will be much more acceptable to them than a longer period of time. Keeping the length of the pomodoro short enough that it is almost always achievable enables a good rhythm, and enables you to make more accurate estimates the longer you use the Pomodoro Technique. Eventually, you basically know how many pomodoros you can successfully complete within a giver time period, and you get a sense of how many pomodoros different activities will take to complete. Once you get good at completing most of your pomodoros and at estimating, you can plan your day well, and feel really good about what you accomplish each day. Shorter pomodoros enable this.
If you feel it might be beneficial, you can alter the length of the pomodoro. If you do this, each pomodoro should be the same length. But the recommendation is to give the 30 minute pomodoro a good chance. If you do change the length, monitor the effect this has and be open to changing it again, monitoring the results with each change.