Hello Jacquelina,

That looks really sore. It doesn't seem to be in your armpit and so unlikely to be a hair follicle infection but because of it's shape it looks like a really nasty allergic reaction to the fabric in the edge of your bra or some other clothing.
I had a similar thing from a certain very stiff lacy bra--it looked like it was pinched and burnt.
I would use Aloa Vera to start with and cover it with some soft gauze to stop it rubbing and then if it doesn't work perhaps your doctor may need to prescribe a steroid cream or something.

I do hope that it gets better soon as the extra pain from that is just not right!
Take care
Tayla