Interest on top of interest only mortgage

Can anyone shed some light on this. I have spoken to my lender and I'm still no clearer.

I have a interest only mortgage. I have been in arrears over the years and have overpayed each month to clear the arrears. I have gone through my statements now my arrears are cleared and have noticed that the balance on my mortgage has gone up by around 10k. When I saw the statements before with Interest amounts, I assumed this was the amount of interest charged as rates vary and this was what my payments were being put towards.. it turns out it's just interest they have been adding over the years..as much as 41/2k per year for no apparent reason. I asked my Mortgage Provider and they have just said it's interest and arrears but I don't have any arrears as I've paid them off so why has't this been deducted from the balance owing and couldn't say what the interest was for....

I'm really confused as to why I'm paying additional interest on top of my interest payments.. is this right... It means I owe more now than I did the day I got the mortgage..

Any advise please