I am extremely worried about my periods. They have dwindled away to nothing and this month I didn't come on at all. I'm definitely not pregnant, because I haven't had sex since I went to Greece at Easter. And in any case, I'm on the contraceptive jab.

Well, that may be the clue to your lack of periods. Many women don't realise the jab, also known as "the shot", does have quite an effect on menstruation. It can make periods heavier but very often it does the opposite and removes them altogether. You should certainly check with your GP but my guess is she'll tell you it's the jab that has temporarily abolished your monthlies.