Reddish Bumpy Patches in Body..

Hello mummies, ftm here.. my baby is 4 months+ and having this kind of reddish, slightly bumpy patches in face, legs, chest, hand and leg area. These patch dont last for days. They just come & go. If today morning appeared some patches, they are more likely gone by tmr. But keep on appearing from different different places of the body daily. Not insect bites. Not itchy. I think this is some allergy reaction but unable to rule out for what. Baby is in formula and he has been drinking the same formula ever since his birth. Showed to PD too but he is also unable to figure out what it is. They are not baby acne also. These patches are bigger than acne and in different different sizes. But on additional note my baby is under antibiotics as he was diagnosed with UTI. These patches appeared like after one month of antibiotics started. So i dont think these are medicine allergy also. Pls help to share if you have any similar experience. Thank you! Posting a picture for better clarity.