Had this mark under my boob for a month now.

I’ve had this mark under my right boob for about a month now. It started off as a sore (first picture) this was quite red and hurt but now has healed and is just a darker mark. I went to the doctors about it whilst it was sore and they gave me some antibiotics which made it heal but the mark is still there... anyone ever had this and know what it is? I’m 23 weeks pregnant and know a lot of changes in your breasts are normal during pregnancy but I can’t help worrying.