Aww mamsh, based sa description mo (dark areas with diamond or square patterns), sounds like it could be eczema or maybe a heat rash, maraming babies dito sa Pilipinas nagkakaroon especially sa humid weather at kung nagsasweaty sila. One of my mum friends' baby had something similar and nag-improve lang after she switched to cotton clothes at frequent baths with mild soap, pero talaga best na check mo na with your pedia para sure kung ano exactly at makakuha ng proper treatment. Don't worry, most skin stuff sa babies ay super manageable once na alam na kung ano!