Sore skin on left shoulder for about 3 weeks, what could it be?

[MarySPH]

I am 23 years old, healthy and fit.
I have had 2 moles removed in 2014 as a precaution, they were absolutely not alarming according to my dermatologist. But due to their location the bra strap area, he decided to remove them to prevent possible complication later on in my life.
I have never been diagnosed with skin cancer nor melanoma and my family has no history of skin condition except the fact that we all have lots of moles that need to be checked at least once a year.
This is my background, quite a lot of moles running in the family, but none of us has ever been diagnosed with anything serious.

After coming back from Oregon (US) to London, in October (where the weather was cold and wet in both countries) I've started to feel a quite sore bit on my left shoulder. It hasn't left me for the last 3 weeks and I'm starting to get worried. The sore spot is about 5cm2, is always located at the same spot and the soreness hasn't changed intensity in the last 3 weeks. Nothing in my life, or in my diet has changed except the fact that I've stopped taking my birth control pill which I had been taking for about 8 years.
The sore spot itself, presents no rash, no moles, no redness, nothing that my eye is judging as alarming. However, I am getting concerned and would like to know if anyone has heard of something similar, and what it could be?
Thank you for your help.