Rash on back.

So a few days ago I was wearing my bra and started feeling like maybe my bra strap was to tight so didn't pay it much mind but yesterday It started to hurt more so I looked in the mirror and noticed this. It's on my upper back right in the area where my bra strap goes. I thought maybe it was a heat rash but it hurts and doesn't really look like any heat rash I've ever seen and it's nowhere else on me. I'm 23 weeks pregnant. Any one have any ideas? I plan on telling my doctor when I go to my appointment but wanted to see if maybe someone had an idea on what it could be. Could it be a heat rash? I have been working outside lately.

Excuse my stretch marks lol