Aww first time mom worries, alam ko yan! Honestly, rashes sa batok ng babies ay common, could be heat rash, diaper dermatitis na lumalaki, or even just friction from their collars, pero since you have the pedia appointment on Tuesday, perfect timing na to show her the photos directly kasi makikita niya in person kung ano talaga!
In the meantime, keep the area clean and dry, and avoid tight clothing or anything na nag-irritate sa area na yan. A few of us mums learned na sometimes it's just sweat or products na hindi bagay sa sensitive skin nila, but better safe than sorry, let your pedia check it out talaga para ma-ease your mind!