S/P sclerotherapy of labial varicose vein

What your were told is exactly correct.  Sclerotherapy will cause some veins to clot and the trapped blood within the vein causes the hard "lump".  This can be drained by popping it as if popping a pimple.

What you are experiencing is a normal part of the healing process and is nothing to be concerned about. When we perform sclerotherapy, the larger a vein is, the greater the potential for there to be trapped blood in the vein as it shrinks down, which is essentially a small clot. Your body will reabsorb this all on its own, but due to the volume it may take some time. To speed up the process we can help reduce the load of the trapped blood by performing a needle aspiration, where we poke a tiny needle into the vein, and try to express or flush out the old blood. Using hot, moist compresses will also help, and certainly using over the counter NSAID's can reduce any tenderness.