Key in your IC number and e-filing password, and you can choose the year you want to check on the left hand side after logging in. From here you can see what was assessed and what was paid/refunded.
If you want a more detailed analysis, you will need to go to the IRB Collections department to speak to their officer who can help you look into more detail.
I've called the customer service department (which is really really hard to get thru at this time of the year) for verification. According to them, they aren't sure why but have escalated this problem.