What would cause by body to become more sensitive to warfarin?

I started on warfarin Nov. '09 after my second round of DVT's and PE's. My starting theutic dosage was 10mg everyday except Tuesdays and Thursdays which I took 15mg in order to keep my INR between 2.5-3.5. This summer, my INR has been tested and been high on 4 separate occasions ranging from 4.5 to 7.9. Today it was still high at 4.2 and we have already lowered the dose to 4days 10mg and 3days 5mg. Today, we lowered it to 4days 5mg and 3days 10mg to see where it drops to over the next 2wks. I have not seen a specialist about the reasoning for the dosage needing to be reduced so much sense the end of July. I only have seen the anti coag clinic and its pharmacist. I am curious what could be the causes of my body becoming more sensitive to warfarin. This is almost an 40% reduction in my dosage. Than just seems like a lot to me and should I be more worried and concerned?