Take bleeding in the middle of my cycle, what can it be?

Doctor, I've been taking the Thames 20 for over a year and now I've started having a mild bleeding in the middle of my cycle. I still have more than 10 days to finish the cartelae and I am 4 days ago with this bleeding. What can it be?
Expert answer:

Escapes are common in some types of contraceptives and often cause for the contraceptive exchange.

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