Hi there,
It looks like a heat\sweat rash, I used to get them all the time until a nurse during a check up gave me this advice and I haven't had it since.
Give the area a good wash using an antibacterial soap - don't use anything with a moisturising base. The antibacterial soap will help remove any bacteria and help dry out your skin. (I use imperial leather as it reminds me of my dad)
When you get out off the bath use two towels. Wrap one towel around you, but underneath your breasts - this will help dry the area completely. Use 2nd towel over the top.
Once the area is completely dry, use powder to again help keep the area dry. If I'm going to bed I don't use anything else. I use baby powder.
Then get a stick deodorant (I use Dove original) and apply it under both breasts before you put your bra on. DO NOT USE an alcohol based one, I promise it will sting so bad as I learnt.
I also use this on my stomach apron from C-sections. The key is to create a barrier to stop the rubbing which causes the rash.
I hope this all makes sense as I'm not very good at explaining things quite right because of my autism so I apologise if I have upset anyone.
Good luck op I hope it gets better soon!!