What Causes DVT While On Chemotherapy For Colon Cancer?

Hello. 8 months ago my mother had a surgery for non metastatic (according to the biopsy kai the rest diagnostic tests) colon cancer. 10 days after the surgery she felt pain on her right foot which prooved to be deep vein thrombosis. She took clexanne treatment right away and a month after she started sintrom and had a 5 days INR test in order to determine the proper dose. The INR value was tending to 2. Meanwhile, her onchologist desided that she should have an adjuvant chemotherapy, just because of the tumor size. So, before starting the chemotherapy she stopped sintrom and strarted having clexanne injections again, as her doctors proposed it as a better combination with the chemo. The chemo treatment (it was given intravenously and was cobined with xeloda pills) finished 6 months later with very little and the CT and the rest of the tests showed total cure (at least for the exact moment). So, the clexanne treatment continued all this period and after the good news we decided to go back to sintrom. For the last 25 days she takes INR tests every 4 or 5 days but it rises really slowly. Especially the last week after 4 days of 1,5 4mg pill per day it only riched 1,6. So her doctor suggested that we go back to injection and particularly innohep 0,7 this time and have a blood test in 2 weeks.
Please, could i have your opinion? Why does her body reacts that way to sintrom? And why it was reacting normally before the chemo? Could this be a symptom of anything that i shoud worry about? Could this be a chemotherapy side effect? Should she take more tests or different treatment? I thank you in advance.