definitely not alone... though mine are just irregular, full stop - not connected to stress.
look at it from the bright side: yes, stress might well be the cause for the irregularities. but, it goes both ways - the irregular periods might be a cause for your current PhD stress(de-motivation), too. rest assured - the period will come, your mood will change once again, and you'll write up that thesis like nothing
but from my experience, it's usually not worth it forcing myself to work when it just won't work. rather it just gets worse cause it adds guiltiness. so, instead, use the de-motivated time now to go out, enjoy the sun, rest actively (a spa perhaps? a impromptu bicycle tour?) and when the mood turns, get back to work with fresh energy!