For about a month now I have had a very painful burning in my vagina entrance after sex. The burning starts after I have gone to the toilet which I do straight away after sex and lasts about 20 minutes.
I am on the pill and therefore don't use condoms. I have only had this 1 sexual partner and have been sexually active with him for a year.
I had thrush just before this problem began and again a week ago.both times I took a canesten tablet and abstained from sex while it cleared up.
I really want to get some knowledge on what this could be as my partner and I have a great sex life now he feels bad every time because it leaves me in pain. Please help!

There are many possible infections. Anyone on line can make a wild guess. But you will need to go to the doctor and get a medical exam. That is the only way, you will know what it is. And what can be done.

Prevention is better than cure. So you better consult a doctor now.