Are you on STATINS too?
A few months ago my GP urged me to start taking statins to help keep my Cholsterol low. As soon as I started I found that the daily dose of Warfarin needed to keep me near to an INR of 3.5 fell from 9mg to 8mg.
A week or so ago, due to a medical scare, the doctors asked me to increase my Statin daily dose from 10mg per day to 40mg per day which of course I did. Immediately my Warfarin dose decreased further from 8mg per day to 7mg to maintain my INR at 3.5.
I take Atorvastatin rather than the more common Simvastatin as it interferes less with my INR so the effect on those taking Simvastatin could be even greater. Do you take Statins? If so, don't worry but do keep an eye on your INR.