It's much easier to diagnose while off the bike. Assuming this is a cup & cone bottom bracket (rather than a sealed bearing unit), it's either too tight, too loose, and/or needs fresh grease.

1. Too loose: You can feel the cranks wobble during your pedal stroke. May also hear a knocking noise.
2. Too tight: This is easier to feel with your hands while off the bike. You will feel some friction (grittiness) when turning the crank. May also make a grinding sound while riding.
3. Needs grease: Again, very tough to tell when riding, and sometimes hard to tell when feeling the BB by hand. Usually have to open up the bottom bracket see the grease.

The other potential issue is with the crank arm. Sometime the flats on the crank arm (where it interface between with the bottom bracket spindle) becomes deformed. If this happens, it will start to loosen on the BB spindle. You can re-tighten it, but once deformed it usually means a replacement crank arm is necessary.