Could be a number of things. You took some time off. Your shoes may need replacement. Your lower leg muscles may not be conditioned enough to run just yet. Remember when you jog, you impose a load of up to 4 times your bodyweight on your legs. You could address each of these issues and either fix the problem, or find the real culprit.
Do you run outside? If so, check how FLAT the running area is. If the area’s tilted (for example, left is higher than the right, etc.), it’ll put undue stress on your body. It happened to a friend of mine.
I had exactly the same problem, just a few months ago.
As I found out, my tibialis anterior was simply too weak. After strenghtening it about two weeks I didn?t cramp anymore, but as it was still too weak and I was running too much, I got something called shin splint. I probably should have cut down my running a few more weeks.
To train the tibialis I did reverse calf raises with only my bodyweight on ab thick board (about 15cm) for 20-50 reps. I did this exercise every day 2-5 times.
Probably you find a better way, but this is what worked for me.
do a google search or search on rec.running for “shin splints” and “shin pain” and “calf pain” even though it’s the sides of your leg (not what you normally think of as the shin) you will get tons of info.
Having been a runner for a good portion of my life, shin splints are inevitable. Beginning of every season I’d get them. Mainly because we used to train indoors on concrete floors. They’re hell on your legs. Primarily you need to stretch your calves and shins properly. Then find a level, soft surface to run on (either a grass field without gaping holes, or a rubberized track is perfect). Ice your shins after you run until you can start running without getting these pains (it’ll take several weeks/months)