Burning sensation during peeing could be de due to UTI. But othet causes like stone disease, STDs, malignancy should be ruled out. Get Urine routine and culture and Ultrasound KUB done. If infection is there treat it with culture sensitive antibiotics. If ultrasound shows stone in bladder or lower ureter, it needs surgery or medication. If u hv a history of discharge per urethra and recent contact, u need to treat Sexually Transmitted Disease with appropriate antibiotics. Red bumps on butt could be due to several viral or bacterial causes. Visual diagnosis is a must. I suggest u to meet dermatologist for this.