I have just been to a clinic in London, where they told me I have something called "endometritis". I don't understand what it is, and when I looked it up in the dictionary, the explanation didn't sound right.

I'm not surprised, since endometritis is usually a disease of horses - not women. It's an inflammation of the mare's womb. I'm sure what you've actually got is endometriosis. This is very common indeed, particularly in the over-30s. What happens is that bits of womb lining turn up in the wrong places in the woman's body. Each month, these pieces of tissue try to menstruate but they can't, because there isn't any space for them to bleed into. The usual result is intense pain. For more details of symptoms and treatment, have a look at the website of the National Endometriosis Society: www.endometriosis-uk.org.