What Causes Blood Clots After A Hip And Knee Surgery?

I have blood clots due to hip and knee surgery. I am 67 y.o. I am taking warfin 2.5 mg at a dose of 1.5 for 5 days and 2 for 2 days. My INR is 1.7 at the moment. I keep a nasal drip and I would like to know if that indicates a complication due to blood clots.