What Is The Light Brown,slightly Raised Spot That Has Appeared Over Night?

Hello. Overnight, a large, asymmeticral, light brown spot has appeared on my arm. It is smooth and has the same texture as the surrounding skin, but is slightly raised (more like inflammation from beneath.) It has a pink border, some of which has also turned light brown. I do not have any fever. I did get bitten by a mosquito, but I cannot recall where on that arm. It looks like it may be a burn, but does not feel like one (is only mildly painful when touched and seems to hurt only beneath the skin.) I also have not been burned recently. Please let me know what this may be.