I have numbness in my pinky. What could be the cause of this?

Many possibilities.. Sensibility to the little finger comes from the ulnar nerve. This can be compressed at a variety of sites from the neck to the palm. There are other possibilities as well. If persistent, see a hand surgeon.
Ulnar nerve issue. Most likely, you have cubital tunnel syndrome, which is compression of the ulnar nerve (the "funny bone") at the elbow. Do you notice that the numbness is worse if you flex your elbow for long periods of time? Do you have elbow pain on the inside of your elbow? Is your ring finger numb as well? Generally, this can be treated conservatively by avoiding excessive elbow flexion. Hope this helps!