So recently I have finished a project that took me a little more than 20 days, which was a small C library with 43 small functions that "copied" original ones like strlcat, strlcpy, memcmp and such. When I went to hand in the project yesterday, I realized in shock that I had no clue of how my code worked anymore. Surely enough I could remember what each function was about, but just by looking at it for a fee seconds wasn't enough to make my own logic clear to myself; I'm not talking about a lot either, just around 30 lines of code per function or something.

So my question is: Is this kinda stuff normal or am I failing at some point? Are there strategies I can take to improve this and actually remember what I wrote?

I'm asking this mostly because I'm still learning the roles and my school understands that if you can't explain your code to someone else, you're cheating, and I'm scared shitless of receiving that kind of feedback even tho I rarely look at other people's code to help me.