Hi. The epididymis, a coiled tube which is part of the male reproductive tract, can sometimes be felt as a small "lump" on top of the testicle. What you discovered on your left testicle may just be your epididymis, so maybe there's nothing to worry about.
On the other hand, the epididymis itself can develop cysts or an infection, and get swollen (called epididymitis). This can also present as a lump in your testicle. But often, if you have epididymitis, you're going to feel some pain in your testicle. Another possibility for the lump is a varicocoele. This is a clump of blood vessels around the testicle which become swollen. Testicular cancer is also something to be considered, although it occurs less often than either epididymitis or varicocoele.
What you can probably do is to observe that lump for the next 2-3 months for any changes. Does it increase in size? Do you start feeling pain in that area? Feel free to consult with a doctor if you note any changes during that time. If the lump really bothers you, it may even be better to go see a doctor right away.
To be more specific, it's more like a pimple, not a swelling. Like a small piece of rock on the testicle.