Sudden hyperpigmentation on arms only - What would be the best treatment to address this and how can I prevent this? (Photos)

I have always had very clear skin on my arms - I mean flawless - and I am not susceptible to freckles. I’m not a sunlover and I wear spf50 daily all year around . Just in the past year, my arms have slowly become covered in freckles, red marks and hyperpigmentation. This seems to be due to very rare accidental exposure - which I cannot understand as it seems to be very excessive. On rare occasions over the years, I’ve got on holidays to hot countries for a week or two and never experienced anything similar. Why would this happen so suddenly when I have barely even had a freckle in the past? I live in a cold country with very mild Summers. For example, the following pictures contain new marks that have appeared literally overnight. This seems to be only happening on my arms. I wear sleeves most of the year and work in doors. I am in my 30s but feel my arms look older suddenly. What would be the best treatment to address this and how can I prevent this from happening in the future? Thank you in advance