Yes I Have Had Swelling In One Foot And Ankle

Yes I have had swelling in one foot and ankle for months...my doctor prescribed
Triamterene HCTZ but I was told it lowers blood pressure... problem is I do not have high blood pressure????
And now I'm gaining wieght. Is this medication right for a person with low blood pressure?
And what could be the cause of the swelling in my foot and ankle?