Irritation from sclerotherapy; can you tell me what it is? (Photos)

Thank you for your question in regards to sclerotherapy. The photos provided are slightly difficult to suggest what caused this irritation. It may be from the compression stocking rubbing and possibly pinching around your ankle, or some of the injected solution was not injected into the vein. To be sure what is best for you, see two or more board-certified providers in your area for a complete evaluation to make sure you are a good candidate and that it is safe for you to have treatment. I hope this helps.

My recommendation is for you to follow up with your treating physician as this might be an adverse reaction post treatment, specially if the symptoms are worsening. This way you can prevent further complication. 
Best, 

Your pictures are terrible as they don't show perspective. Is that the top of the foot near the ankle ? Anyways, you need some topicals to reduce hyperpigmentation. You need to use some adjunctive measures after sclerotherapy to reduce post inflammatory hyperpigmentation. it is those that don't follow such instructions immediately following sclerotherapy, starting on day1 (next day) that seem to get retained hyperpigmentation. I recommend 2 creams: Scleroquin plus and Sclerovase and also Scler-X supplement to reduce post inflammatory hyperpigmentation (see link below) as well as compression stockings for at least 2 weeks.
H Karamanoukian MD FCS
certified vein specialist from ABVLM and double board certified surgeon

The pictures are not easy to evaluate.  You seem to have irritation from the sclerotherapy with localized redness and an inflammatory reaction. Hopefully this will subside with localized heat to the area and ibuprofen.  If it worsens, you may form an ulceration from the injection.  Follow up with your treating physician.

Although difficult to appreciate what is happening from the pictures it is possible this is from sclerotherapy injection that did not go into the vein.  The sclerosing agent will cause an ulceration after the skin breaks down which generally happens 1-2 weeks after.  I would have the treating physician re-evaluate the site if this was the site of original injection.