One of the key features of borderline personality disorder is emotional instability (also called emotional lability or affective dysregulation). People with BPD experience a lot of dramatic shifts in their emotional states. They may feel okay one moment but feel extremely angry, sad, lonely, afraid, jealous, or shameful moments later.
Having a lot of emotional changes does not mean you have BPD. First, emotional instability is just one of the symptoms of BPD. To meet diagnostic criteria for BPD you would have to also have some of the other symptoms (e.g., impulsivity, sensitivity to abandonment, self-harm or suicidality).
Also, the emotional shifts typical of someone with BPD are very intense and frequent. Everyone has emotional changes every now and then. But, people with BPD experience mood changes that are very dramatic and very severe, so much so that they significantly impact work, school, relationships, or other areas of functioning. They also happen consistently (more than just every once in a while) and over the course of years.
Finally, in addition to the mood changes, people who have BPD also have other problems with emotions. For example, they tend to feel less positive emotions than other people. They also report chronic feelings of emptiness, or times when they feel nothing inside.
Maybe you are someone who has the tendency to be emotional and are wondering if you might have BPD. If your emotional changes are not getting in your way (i.e., they do not cause you problems and are not particularly distressing for you), it may be that you are just a sensitive person. But, if your emotional changes are causing you a lot of problems, it can be helpful to see a professional for an assessment.
American Psychiatric Association. Diagnostic and statistical manual of mental disorders, 4th ed, text revision. Washington, DC: Author, 2007.