I'm having pain in my lower part of my bladder. What could be the cause?

Infection OR other. Midline lower pelvic/abdomen pain may be due to bladder spasm (due to infection) or uterus cramping. Other possibilities are bowel related: constipation or inflammation of lower colon (?Appendicitis or colitis). If pain is severe or prolonged with fever, or vomiting or 'blood in stool'--it is time to get seen. Hope you are better soon!