feel like jelly
To feel very unstable or weak. This phrase is often used to describe the knees or legs. My legs felt like jelly when I got off that roller coaster—I had to sit down right away.
be/feel like ˈjelly
(also turn to ˈjelly) (of legs or knees) feel weak because you are nervous or frightened.: She couldn’t move — her head was swimming, her mouth was dry and her legs felt like jelly.