EVLT 12 weeks back, Now I am feeling warmth and sometimes burning pain when I stand or sit. Is this issue to be worried?

It’s a little unusual to have it this late. I would recommend going back to your vein treatment provider and having him look at it. It’s likely not anything too serious, but not something you want to ignore. Best of luck!

If you are having delayed pain, it may be simple late inflammation (phlebitis), which is possible. It is important to have an ultrasound of the area to confirm that, exclude a deep vein thrombosis (DVT or blood clot), and make sure it's not something else.

Most discomfort following EVLT is resolved by 4 to 6 weeks and most within 2 weeks.  You may be feeling some nerve irritation from the laser procedure or you may have another associated problem such as cellulitis. It is difficult to assess your problem without a picture or seeing you.  I would recommend ibuprofen and wet compresses to the tender areas and, if this does not help within 1 week, then follow up with your treating physician.