Developed a Hard Lump After Schlerotherapy

After a clot is extracted a firm lump can persist for weeks. In order for you to feel more secure, you should await resolution of your concern and then proceed ahead.

If 'hard lumps' occur after sclerotherapy, they can be drained through a small puncture using an 18 guage needle and drained effectively with excellent cosmetic outcomes. As you say, this can happen if the injected vein is too large and more likely to have happened because of inadequate compression following sclerotherapy. 

Sclerotherapy involves the injection of a solution into a vein to dissolve the vein. In the larger veins, the blood inside the vein may clot after being treated. This will usually resolve over time. Warm compresses and NSAIDS may help to expedite the resolution of the clot. Lancing of it as you had will also help but this may need to be repeated.