urine infection?

I had a urine infection a couple weeks ago and came off of 4 day antibiotics on the 25th, and everything has been fine until today (27th) my pee started to burn again and I noticed some specks of blood this time, which I didn't have before. Is this just another urine infection or could it be something worse?
 I've also been experiencing back pain from the top of my waist right down to my tail bone. I saw my doctor about this and she very quickly said I'm simply not stretching enough, but it doesn't always feel like muscle pain. 

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