Why do I have a constant need to urinate but can't excrete more than a small amount of urine at a time?

Possible UTI. There are several things that can cause decreased bladder emptying. The most common is a urinary tract infection (uti). There can be some more concerning things as well. Your doctor needs to be notified about this change.
? Bladder empty. Strangury is symptom of constantly needing to urinate even with an empty bladder and can be caused by urinary tract infections. However if symptom persists needs to be evaluated medically to rule out bladder tumor, bladder stone or partial urinary retention from conditions like cystocele.