Pleasure is associated with the word orgasm but for a small percentage of women, it unfortunately brings nothing but pain.
Without knowing your medical history, I couldn't begin to suggest what the cause is so I highly recommend that you make an appointment with your GP or gynaecologist to be checked over.
I don't know if you have always experienced pain with orgasm but many factors such as childbirth and related issues (scar tissues etc.) and hormonal changes can certainly affect your physical reaction to orgasm. For example, if you are pre-menopausal, the drop in hormones may cause your vaginal wall to become drier and sex may be less enjoyable overall.
I'm also not sure if you are referring to orgasm during intercourse in which case, the pain may be due to the position of your partner's penis. You can certainly try some simple things like using lubricants to see if it makes any difference for you but ultimately, I really feel it's worth being examined medically first. You deserve to enjoy an awesome sex life and the solution may be very simple.