I Just Adopted A 5 Yr Old Rescue Female Dog

I just adopted a 5 yr old rescue female dog that was spayed a few days ago. She is urinating frequently and there appears to be fresh blood in her urine. The shelter assured me that this is normal after the surgery. They gave me an antibiotic called Enro 68mg, 1/2 tablet,to be given once a day for 12-15 days.,