Could Swelling In Thighs Be Due To Clotting Of Blood?

I have anal cancer. Treatments with mytomycin and 5fu were insufficient to arrest it. Spots biopsided on my neck show squamous cell carcinoma. Also showing spots on CAT scans near kidneys. I am on 4th cycle of Xeloda. My left thigh swells to great hardness. Have had sonogram in the area to rule out blood clot. Could there be a clot elsewhere? Is this a reaction to Xeloda? Is it Ok to take long walks if I CAN. SHOULD I BE SERIOUSLY CONCERNED WITH WHAT THIS MIGHT BE A SIGN OF? THANK YOU