Red spot on back of my neck for 2 days has grown increasingly sore, feels like a zit w/ fluid, but it's not. What is it? (photo)

It doesn't appear to have grown in size, but it has started impacting my entire neck, which is now sore. Any thoughts on what it might be? There is no clear "head", it is lumpy to touch, bothering me more and more. Doesn't itch. Thanks for your help!

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