Fluid retention

Hash

Any advice or help on this one would be hugely appreciated. I've been running for years and never had this problem before, but it seems to be happening more and more often. I've got hideous fluid retention, which all seems to be in my legs as they're huge, whereas my top half is the same as normal. I know it's fluid, as whenever this happens happens I put on over half a stone in two days, and even if I ate nothing but beer, chips and mars bars I'd struggle to put that much fat on that quickly! Also the skin on my legs is red and itchy, but I think that's just a skin reaction from running in the cold, even with my winter long tights on. I drink loads, it's all decaf coffee and water, but probably more diet coke than is good for me. I don't eat a huge amount of salt, I've been to the doctor, and blood/urine tests are fine. The only change from last year is I've started taking the combined contraceptive pill, and the problem is generally more noticeable in the pill free week, but I've now had this latest bout for over a fortnight, so that's that theory out of the window. If anyone has any advice or suggestions, I'd be hugely grateful.