Pain after sex

18 months ago I suffered a miscarriage. It was twins and the miscarriage took 12 weeks from start to finish. Since then I have suffered moderate to severe pain after sex and also when I need the toilet. The pain is a constant sharp ache and it feels like I have been pumped full of air in my abdomen. I also suffer from vaginal cramps from time to time. I went to see my gp about this about a year ago. She sent me for a scan, did a smear and checked me for any infections. There doesn't appear to be anything wrong with me. Obviously this is making me really miserable as I am married and don't feel we can live a full life together. I hve mentioned it again to my gp but she shrugged her shoulders and brushed past the subject. If anyone has any ideas of what might be causing the pain I would be very grateful for them. There must be something wrong because I have never had any problems in the past, just since the miscarriage.