From your past medical history of Stroke and Left hip replacement , the probable cause of your swollen foot and ankle, is Deep Vein Thrombosis (DVT , in short).
DVT is a condition in which there is formation of a blood clot ("thrombus") in a deep vein and it usually affects lower limb veins. This clotting eventually leads to swelling and redness of the area of limb where the vein is clotted, as in your case it is your foot and ankle.
Though DVT is the first diagnosis that comes to mind , howevwer there may be second diagnosis and that is Varicose veins.
Now to confirm the diagnosis of DVT there are few main Investigations that can be carried out. These are-
2)D-dimer blood test
3) Doppler ultrasonography
These are a few tests which can confirm your diagnosis.
So i would advise you to seek help from a Physician and mention all the possible investigations that i have suggested you , so that he chooses the best and the surest out of the lot.
Please do not take this condition lightly and seek help from your doctor as soon as posible as if it is DVT then it has a life threatening complication in the form of Pulmonary Embolism.
As far as treatment of DVT is concerned , your doctor after the mentioned investigations are done will decide whether hospital admission is required or not. Usually Blood Thinners (Anticoagulants) are give in this condition.
Wish you good health.
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