Hi mama, first of all.. I’d like to say you’re doing a good job.
I understand your concern about your baby’s weight but I need you to know that babies are different and thus grow differently. The important thing is that your baby is eating well, playing well and pooping well. A healthy baby is all that matters.
Secondly, babies below 6 months should only be given breastmilk and/or formula, the complan is totally unnecessary.
Third, rashes are mostly caused by heat, you can reduce clothing or use agnesia powder from 2 months, babies enjoy good air too, so unless the weather is chilly, they don’t need to be wrapped up like shawarma.
Fourth, never administer any unprescribed drugs to a baby, even yourself as an adult shouldn’t take drugs the doctor didn’t prescribe for you after due tests have been conducted. Most especially antibiotics, it’s really not advisable. It kills both good and bad bacterias in the body and I doubt bacteria has anything to do with rashes so it’ll kill all the helpful bacterias in the baby’s body. The good bacteria’s help fight the bad ones against infections. So when there’s no good bacteria to fight, the bad ones that come in can cause an infection, I hope you understand what I’m trying to explain?
Nevertheless, The hospital is a good idea, always seek professional help. All the best.

She's not underweight. My baby was 5.9kg at 3months 3weeks and few days.