What Causes Pelvic Pain And Uti With Constipation?

Please can you advise me. I periodically get pains in my pelvic area, which are increasing in regularity. I have an increased urge to urinate and I feel that I have the start of a UTI, although I have recently had a culture taken that showed no abnormality. The pain in my pelvic area is quite severe and lasts for about and hour then diminishes. This pain is really beginning to worry me as I never know when its going to happen. I have noticed that it may be worse after drinking alcohol. At times I am constipated and I also think that this may make the pain worse. I am currently dieting although finding it difficult to lose weight. Thank you