Have someone get some tweezers and see if there is an ingrown hair and pluck it out. If it's that, and the hair is out, wash it with soap and water then apply Neosporin a free times a day for a few days.
Could also be a bug bite. I see other pink marks like bug bites but one must have become infected.
If it from the clasp on your bra digging into your skin?
Put an anti itch cream and Monitor. If more start to develop, consider bed creatures. If I can rule those out and still getting more, seek doctors help. I’m glad to hear your chest X-rays went well.