Have you got your mortgage statements showing transactions / balances?
When you are on interest only, the amount of actual interest charged is based on the number of days in the month. In your case, 188.48 in Feb, or 209.78 in January.
The bank tries to even it out, and collects about 206 each month, so there will be small differences, that should balance out over time.
Another place it can go wrong is when there's a change to your rate, but they can't change the collection amount in time - for example you must receive 14 days notice of a DD amount changing, but typically a tracker rate must be changed within 5.
When you're paying capital as well, these differences get lost, but are obvious on interest only. A difference of 3 months interest is very high though, so you'd need to analyse your statement to see when it started.