Classic Introverted Overthinking.

When someone you know gives you a present out of the blue and it's NOT holiday or special occasion, it's easy for introvert types to retreat within themselves and try to "figure out" this complex puzzle of "why". Analytical thinking is one of the introverts greatest gifts. It can also be our greatest hinderance as it can lead to unnecessary overthinking.
Relax. Breathe. Sometimes people just like to show their appreciation for you. That's it....weird, I know.