If your mast is deck stepped that shouldn't be the leak. But if the boat is covered well enough to keep the deck dry, you may have to blame the deck step. Either way - you need to locate the leak to at least know what you're dealing with..
If you see and feel no visual evidence of the source of the leak, I suggest you start at the end and work your way back. Be creative. You could snap some chalk lines near the top of your bilge to see where the water is entering the bilge, then work your way "up" to the source.