Sclerotherapy works by clotting the veins into which the solution is injected. On small veins the sclerosing solution pushes the blood out of the vein causing the walls of the vein to stick together and then dissolve. On the larger (varicose veins), clots can form in these veins and become tender. You may have developed this or possible extension of the clot into either the small saphenous vein or even the deep veins resulting in either new superficial phlebitis or worsening of your DVT. You should have a venous ultrasound to evaluate these veins. If this is normal then the discomfort should improve over time. Wet warm compresses to the area may help.
It is not clear why you got a venogram before or after sclerotherapy. If you have had a DVT, it is likely that you have deep venous insufficiency (versus superficial vein insufficiency) or both deep and superficial venous insufficiecy. You also have post thrombotic syndrome. I would recommend that you wear stockings 365 days a year, rain or shine !
Sclerotherapy is the best done for tiny spider veins once the condition responsible for their formation, venous insufficiency, is eliminated. Under other circumstances, sclerotherapy is: 1. not effective, 2. in your situation, it is also not very safe, since once injected, the sclerosant solution is not controllable and its distribution, therefore, is unpredictable. It means, the solution can get into your deep system through so-called "preforators" and cause deep vein thrombosis. Right now, you should, therefore, check your deep veins in a specialized venous practice. The results of such evaluation will determine what needs to be done next. Hope it helps.