What does it mean if my two big toenails are blue underneath now that i've started wearing tennis shoes and working. they also hurt.

Microtrauma: It sounds like your toenails are hitting the end of your shoe. This usually happens if the toe box is either too small (toe is pressed up against the end) or too large (foot is sliding around and the toe is hitting the end). This is very common in runners--it can lead to either thickening of the nail, or it will eventually turn black and fall off--a new nail will grow in within a year.

Typically: It means you should trim back your toenails and look for better fitting shoes for your work. If it does not get better your doctor or a podiatrist may have some ideas.