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.

is an area of "phlebitis" in the vein and you had that blood removed with a thrombectomy procedure.

that lump should continue to subside over the next few weeks to months but the process is slow.  be patient, it can take many months sometimes for that lumpiness to fade.  this is entirely normal and part of the sclerotherapy process.

Cheers

I strongly recommend you be on compression stockings to minimize this next time. This coagulum (which can be tender to touch) is from blood being trapped in the superficial vein when the vein collapsed from treatment. It is usually seen when reticular or varicose veins are treated, rather than when smaller spider veins are treated.