I had a ct scan about a month ago that showed a 3.5cm cyst on my spleen. today, i went for an ultrasound to see if the cyst had changed sizes, but i was told that the ultrasound showed no cyst. can splenic cysts disappear this quickly?

Yes; but diff modali: The CT scan & ultrasound (US) measure different things. CT uses X-rays which address nuclear density (minerals vs. carbon) whereas US uses sound waves to determine solid vs. liquid. CT uses tomography (math) whereas US uses hand-scanning. So the US may have missed the cyst, or the cyst cd b solid in which case it wdn't have been seen. Depending on cause, splenic cysts do regress. What's the cause?

Yes: Cysts can come and go. The other aspect to consider is that u had a CT scan first which is a 3D picture with more image details initially and for follow up an US, which is reasonable to do as they should have the initial CT can results and can correlate this with the US to make sure they have the same spot. Otherwise, if they don't, then that could be the issue.