Thank you for writing to JustAnswer with your question.
In most cases of deep vein thrombus, there would be some swelling or edema in the leg, redness or inflammation along the vein area (often felt in the back of the knee).
But, sometimes these symptoms don't show up, especially in a young person who may have excellent collateral circulation.
Sitting in one position for a long period can predispose one to this problem, especially if one is also dehydrated - has not been drinking much fluids. This is one reason why DVT may occur to an individual who has been on a long car ride or airplane ride. In fact DVT from this cause is sometimes called "business class syndrome" because the cramped seating in an airplane may predispose an individual to DVT.
Although you sound like you have researched the topic already, this is a good link for future reference:
Due to the amount of pain you have, and the fact that treatment is better to start sooner rather than latter, you should go to your doctor (probably too late in the day) or an Urgent Care Clinic or ER today. They can do some simple blood work and an ultrasound of your leg to check for DVT.
I hope for your sake this is a sore muscle from starting out an exercise plan with a 3 mile walk, but we cannot take the chance that it might not be. I do hope you continue to exercise, drink plenty of water, and get up from your desk every 2 hours at least from now on, no matter what the results.