What Causes Bloating, Frequent Urination, Cramping And Lower Back Pain?

Hi I am 31. I have been having bloating, frequent urination, cramping, and lower back pain for about 10 months now starting about 2 weeks before my MS. I am having the same symptoms now and was due 3 days ago. I took a test and am not pregnant..didn't think I was.. I do have children and have known everytime. I was wondering what would cause this for 2 weeks out of the month? It is worse now than before but my doctor said it was normal. Should I get a second opinion?