PMS symptoms for longer than usual, no period yet, and NOT pregnant. what could be causing this?

Life. Your body is ever changing.

Are you stressing about something?

Inflamed fallopian tube, soft tissue infection - I would see a Dr.

Premenstrual syndrome (PMS) is a group of physical symptoms, psychological and emotional issues that women experience during one or two weeks before a menstrual period. These symptoms disappear soon after the onset of menstrual bleeding.

PMS may be a chronic condition. In some women, PMS symptoms intensify before each menstrual period and this pattern continues until menopause. In other women, PMS symptoms seem to diminish after 35 years.

Even doctors do not know the exact causes of PMS, there is no way to prevent it. However, you can relieve some symptoms if you have a healthy lifestyle.
# Begin a regular exercise program (three to five exercise sessions per week).
# Do not avoid meals. Follow a regular feeding program to maintain a stable level of blood sugar.
# Eat a balanced diet low in refined sugars.
# Try to sleep well at night.
# Do not smoke
# Reduce caffeine, alcohol, red meat and salty foods.
# Practice stress reduction techniques. Take a nice bath and prolonged or try meditation.

Source:
http://www.herbalcureindia.com/healthcare/irg-menstruation-menopause.html

You might want to look into whether or not it is possible that you have a hormonal imbalance. If the problem persists, you might want to consult your doctor and see if they think you should get your hormone levels tested.