How do u get a swollen fallopian tube?

Prior infection: The most common reason for a "swollen fallopian tube" is an infection, either current or prior. This can be from an std infection.

Infection, scarring?: A swollen fallopian tube can be from an infection, including sexually transmitted disease, or from blockage of the end of the tube causing fluid to accumulate in the tube (hydrosalpinx). This can be due to scarring for infection or possibly endometriosis or prior surgery. It depends on what the doctor means by "swollen tube".