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Could be lots of things - fingers could be saddle tipped down causing weight to slide forward (try that test where you ride on a turbo at threshold and let go of the drops - if you immediately tip forward you have too much weight forward), could be tight gloves or something like a locked out arm position. Feet could be too tight shoes or socks, could be pinching a nerve so you need to do some stretching - I find lots of stretching around the hips helps with a lot of things - lower back, quads, glutes, hip flexors - they're all worked hard on a bike.
I've not had much time on the bike this year (other than utility cycling), so I'm fat and unfit, but on longer rides (3.5+ hours) I've been getting a bit of tingling in my fingers and toes. Only on the left side, though - is there anything obvious that could be causing this? Saddle and stem seem straight, and the bars are centred in the stem.