What Causes Hand Shakes After Lifting Something Heavy?

After doing my groceries, I carried the walmart bags (8kgs per hand) for 4 or 5 blocks and when I rise a glass full of water with my left hand, it starts to shake while holding it.

I'm 36 years old, haven't do heavy exercise in the last 4-5 months. Use to drink a beer or glass of wine with the meal. A 18 minutes walk from my apartment to the movies or mall, accelerate my heart beart.