What Causes Delayed Periods?

I am a 26 year old female. I have not had a period since February of this year and have had lower to middle back pain for about 3 months, When it is really bad it will penetrate down my legs, I have had 3 pregnancy tests done and they have all been negative. At first I think it was a UTI as I have Interstitial cystitis. One urine sample did come back as a UTI and I was put on medication but the back pain did not go away.