Could this be a blood clot or sore muscle?

For the past for days my right calf muscle has been feeling achy on and off. The achyness goes away when I exercise and starts aching again after 2 or 3 hours. Sometimes it just goes away on its own then comes back. It doesn't hurt like "ow! Leg cramp!" hurt, it hurts like "I just finished walking a lot and now my leg is achy". The problem is it feels this way when I'm laying down and resting. Now I'm 20 and I never use to exercise but I resently started exercise, about a half an hour on an old exercise bike on max resistance,(witch isn't that hard) my leg isn't swollen or red or blue, it's not warm and I can feel my pulse on my foot (both around the top of my heel and top of my foot area, it's like a little worm pulsating! so cool!) as I'm writing this I just finished exercising about an hour ago and my leg is feeling really good. I think it has to do with the fact that I never really exercised and now I've started.