Is this thrush or something more serious?

so i recently went on the pill, and for a few months everything was fine. But in the last 4 months or so, i will have a period, then about 5 days later my will become very itchy and extremely irritated, it gets sore and dry. i also start getting a strange, thicker more yellow discharge. this usually lasts between 7 - 10 days, then it goes back to normal until after my next period. This has become like clockwork, is it thrush? to do with my pill? or something else? what do people think?