aching in back - lower left side

two weeks before my period, I started getting intense cramping in the front and in my back lower left side that would radiate up to kidney area. Probably the worst cramps I have ever had. This would continue for two weeks and then disappear. Shortly after, I would get my period. This went on for a few months before every period and then the last time I never got a period, I found out I was preganant. Now I am fifteen weeks along and the pain has returned. It is mild cramping this time but with the same ache in my back , lower to mid left side. I would say in my flank. Any ideas???