I also have a short cycle around the same length as yours. I'm 25 weeks now and they have calculated that I fell pregnant 7 days after my LMP. So it's obviously possible to ovulate early in the cycle because I definitely did! It literally all depends on what is usual for your cycle and how long the sperm survived in your uterus for! I would take another if it's early or maybe take a digital one! I tested about 3 days after my period was due and got a "PREGNANT"
With a 23 day cycle you'd ovulate around day 9 of your cycle theoretically. So I'd still think day 15 would be too early for a strong positive. Maybe you ovulated even earlier? Or perhaps you actually got pregnant last cycle, as you said and your previous period wasn't really a period?
I’ve now taken 4 tests including a digital and got pregnant 1-2 weeks, still in disbelief!
With shorter cycles it's definitely possible lovely or you could just have a high amount of hcg too, some people just do. But yeah if your last period was light and abnormal in that sense for you then there is potential for it to have been implantation bleeding xx