Today I noticed a small lump inside the back of the neck on the left side. What could it be?

Normal lymph node. We have lymph nodes scattered throughout our body & they are often detected in the neck where they sit on top of neck muscles. These nodes are little fluid strainers that take body lymph & remove stray germs and debris from things like bug bites. They are often pea sized at rest & grow to dime sized when working. Within 2-3 wks they often shrink back down unless they are stimulated further.