The causes could be:
1- Diabetes mellitus (insulin problem)
2- Diabetes insipidus: consists of 2 types: a/central: here there's a deficiency in a hormone called ADH, giving messages that you're constantly thirsty even when your body is hydrated; b/nephrogenic: the kidney is not responsive, partially or totally, to ADH, and in adults it's secondary, usually to meds like Lithium
3- Pregnancy: early on, urination frequency increases
5- Cystitis (bladder infection) or urinary tract infection: especially if you have on top lower abdominal or flank pain and burning with urination
If all the above is ruled out after evaluation by a physician (history, examination, and tests), you may have what is called "Overactive bladder", which is a symptom syndrome (not a specific pathological condition) consisting of urgency (involuntary bladder contractions lead to frequent and often urgent urination, meaning you have to get to a bathroom right away even if your bladder is not full), frequency, and nocturia (frequent urination during the night).