I would go see an endocrinologist ASAP.
High prolactin can be caused by several things - but the rapid weight gain could make it associated with high cortisol. You need to get testing. I would take pictures to show the rapid progression of the weight gain.
I had Cushing's disease and a prolactinoma. It would make you gain insane amounts of weight. I also had PCOS too.
I've had high prolactin before and as far as I know it is not associated with weight gain. It is associated with breast discharge and decrease in libido. Also, prolactin is made by the pituitary glad, which also secretes other hormones. If there is a general issue with your pituitary, it could be that some other hormone might be causing the weight gain. Do you know if any other hormones are too high or too low?
I have no idea if prolactin makes you put on weight - but high prolactin is definatley a hormone associated with Pcos. Sorry I couldnt help further