What can this be ?

أليسا

My daughter (5 months) seems to always break out on her back every time we take her outside or if she’s in her car seat for more than 30 mins , seems she gets it more when she sweats but what can be causing it. ? Her doctor doesn’t know what it is and he just gave her Hydrocortisone 1%

I’ve washed all her clothing and the car seat cover twice and we use baby dreft stage 2