Bicuspid aortic valve. ct two months ago to look for aneurysm. now having sharp back pain right below my heart. can an aneurysm develop that quickly?

Generally not: If your ct scan was negative, you have very little to be worried about. A sudden change in an aorta can occur with a spontaneous disection, but your symptoms are not typical for that.

A dissecting one...: A abrupt bleeding into the wall of an artery is called a dissecting aneurysm and can develop sudden/rapidly. This usually cause severe pain. Otherwise, aortic aneurysm--dilatation of an area of the vessel as in picture here, often takes several years, usually in older folks having atherosclerosis/hypertension. At 25, it is highly unusual. Consult doc for other cause of pain. Good luck.