I had a sclerotherapy and 3 days after i have a slight fever that goes on and off for almost 3weeks now. does it have to do with sclerotherapy?

Most unusual: Fever following sclerotherapy is most unusual and to go on for 3 weeks is even more unlikely. There is a small potential for an allergy to sclerotherapy solution but this should not cause on and off fever for over 3 weeks. I would look for another source of the fever. You should return to your treating physician or see a medical doctor for a work up of your symptoms.