Just finished irregular period, moody, pain in lower left abdomen, and frequent urination. Negative pregnancy test. What could cause this?

Different causes. Whenever there is frequent urination you have to think about urine infection. However, other causes of difficult menstruation (dysmenorrhea) are possible including endometriosis, fibroid and hormonal imbalance. Would see family doc or gynecologist. He/she will check urinalysis and explore other gyn related causes also.