What causes frequent urination, little excretion, pain on left side of abdomen and urine culture to be negative?

Cystitis symptoms: Interstitial cystitis can present this way. So can urine gravel / small stones. So can tubal infection with genital / urethral inflammation. Please be checked.

Constipation: If there is too much stool (poop) in the rectum and lower part of the colon, it can push on the bladder, leading to frequent urination. The contractions that try to get the stool out often cause pain. This is the most likely cause of these symptoms and can be confirmed by simple x-ray. Of course other less common things may be present. Discuss with your doctor.

Many possibilities: These can be symptoms of a bladder/urethra inflammation not necessarily secondary to a bacterial infection. In a 20 y/o female if you are sexually active you need to consider the possibility of an STD like gonorrhea or chlamydia. Go see you doctor.