A couple of years ago, when I started publishing in my field, I received several requests of review from 4/5 different journals in a row. Because I happened to have free time, I submitted those reviews (done conscientiously, although in just one or two pages) in 24/48 hs. I know this is super fast. Then nobody has asked for my review in several months. I recently read that there was a scandal, I think in China, where journals discovered that reviewers answering in 24 hours were the authors themselves in disguise. My question is: are editors letting me rest? Were my reviews too short? Or, might they have mistrusted my reviews because they were too quickly done?
In my experience, the unusual thing is not that you haven't been asked to review recently but that you had a batch of review requests all at once.
Most journals that I have worked with, as either editor or reviewer, prefer to use the same reviewer no more than 2-3 times per year, in order to maintain a diversity of perspectives and in order to avoid burning out reviewers. If nothing happens across an editor's desk that makes them think of you, it could be years before a particular journal asks you to review again.
Referrals also play a role in being asked to review. Editors will often use at least one of the reviewers suggested by the authors. Also, if somebody declines a review due to being too busy, the editor will often ask the person declining for suggestions of alternate reviewers.
Thus, my guess is that you had an unusual coincidence of referrals early on, and that you've now reverted to a more normal rate of review requests. If you want to have more review requests, start spreading the word through your professional network, and people are likely to start pointing to you in their referrals.
I am not sure if you made the journals suspicions, but here is what I do when my reviews take extra-short or extra-long time. I let the editor know what is going on. For example, I just finished a long programing project and the referee task was a welcome change. Or, this paper relates to an active project so I needed to read this right away for other reasons. Or, in the other direction, I might say that I found part-way through the paper that I needed to review a math topic I had not worked on for a decade and the paper is fine. Even with this, communication does break down and editors end up mad at me.
I seriously doubt that any editor is going to be much concerned that one review came in fast. This happens sometimes for valid reasons. Just make it a habit of adding comments to the editor to let them know the context. Editors are humans.
I can add a sample size of one from personal experience here:
In my field (somewhere between physics and applied mathematics), the actual workload of a peer review is at most one day of work while the average time for peer review is one month. The main reason reviews take so long is that reviewers need to find time for the review during their other duties (this applies to every field).
When I started reviewing (on my own), I was handing in most reviews within a few days. (The main reason for this was that my other duties were long and flexible, so a task quickly done was a welcome change and whether I did the review immediately or after two weeks would delay my other duties all the same.) I consider my reviews to be thorough and certainly not overly positive.
As a result, I became an “emergency reviewer” for one big publisher. They never officially told me, but I clearly got only sent review requests where the authors had been already waiting an overly long time, e.g., because one of the original reviewers failed to deliver or they needed an additional opinion. Mind that I did not get review requests at a high frequency, but those which I got were only “emergencies”. Other journals continued to request me as a reviewer in a normal manner.
Were my reviews too short?
It’s naturally hard to answer that. At least in my field and many others, reviewers are “trained” by starting to work on reviews collaboratively with an experienced reviewer (usually their supervisor). If you have undergone such a process, you should be aware what constitutes a good review in your field. If not, you might have committed mistakes that you don’t even notice.
Or, might they have mistrusted my reviews because they were too quickly done?
Unless your review time was below the actual workload of a proper review, I doubt that. The usual hallmarks of bad reviews are being overly positive or negative or too superficial. I would expect that journals focus on these criteria, since bad reviews can also take a long time – many people have received useless two-sentence reviews after months of waiting. On the other hand, if a low-quality review coincides with a short delivery time, this should fuel any suspicions and bad impressions.