Retrograde Alopecia? Or Damage From Hair Dye?

Taking note of the symptoms described, this could be a case of androgenic alopecia. Though this ailment is known to enhance hair loss in the front hairline, it can also do the same for the back of the scalp.
Though chemical dyes are not the cause for this ailment, they sure can add to the effects of it and lead to making the symptoms of hair loss and thinning worse.
This ailment needs to be diagnosed using corroborative diagnostic tests. A dermatologist will be able to rule out or diagnose the condition along with preparing an apt treatment plan for the same. An early diagnosis might help in stalling the progress of this ailment and help you understand more about your potential diagnosis.

Your problem is most probably retrograde AGA. Dying does not cause hair loos in a limited arae

There’s more on the list actually and so I would need to see photos and examine the area. Ophiasis, lipedematous scalp, retrograde AGA, and cicatricial alopecia are possible. The timing you describe does not fit well for dyes. Be sure to see an expert. A proper up close examination is needed here.