Worried I may have DVT or a blood clot

So I woke up one morning(4 days ago) and I had pain in my inner left thigh. At first I thought I might have pulled a muscle since I had gone christmas shopping the day before and had been out walking all day. Now 4 days later the pain is still here and is causing me to have a limp at work. I've also developed two large red spots in the affected area which have fever to them. The pain is when im standing or walking. It progressively gets worse through the day. I'm 21, smoke a pack a day, and am unmobile most of the time (sitting at a pc) when im not working. How do I know if this is a groin pull or a blood clot? When should I go to the doctor? Thanks for the help.

[Dr Sharma]

Hi there. Since you have a job that keeps you immobilised for long hours and a recent onset thigh pain bad enough to give you a limp, you must consult a surgeon immediately for a clinical evaluation.  Screening investigations are D-dimer tests, and plethysmographic techniques and definitive diagnosis with venography andultrasonography.  Other possibilities could be superficial thrombophlebitis, post-thrombotic syndrome, chronic venous insufficiency, venous obstruction  or cellulitis. Take care.