A.B.
I was ridiculously regular right up until I had my third child. After I had her and started having periods again, they were really unpredictable. I'm still not entirely sure if it was an age thing or just an unexplained hormone shift or what. But I went from having a very predictable 28 day cycle to having anywhere from 28 to 34 days, with one month skipped entirely each year until I got pregnant with my fourth. In fact, we ended up expecting with him because of the month that skipped (but were planning on having another; it was just a little sooner than anticipated). Since having him, my cycles went to a 32-34 day cycle, with occasionally shorter or longer gaps. It is very frustrating when you have spent most of your life being very regular.
My point is your body can change even if it's always been predictable. Things like stress, a change in diet, a change in your exercise routine, etc. can impact your cycle. While it's weird and unsettling, it doesn't necessarily mean you are pregnant OR that there is something else wrong. Unless you have other symptoms or reasons to believe that the test is wrong or that there is something wrong with you, just give it a couple of months and see if things return to normal. And if your period still hasn't returned, another test and/or a trip to the doctor may be in order.