This has happened to me too! I found that the problem was my shoes were about a 1/2 size too small. Someone correct me if I'm wrong, but I think that your cycling shoes need to be a 1/2 size larger than your regular shoes.

I knew this but ordered a pair online saying "yeah, I'm a 7 1/2" but forgot that when I bought my first pair of shoes in a store, they told me the shoes needed to be a little bigger than normal.

Might want to try changing that. And as far as it only being your left foot, most of us has one foot that's slightly larger than the other...your's might be your left.

I hope that's all it is. Now I have to talk my husband into letting me get new shoes!

I used to get this, when my cycling shoes were worn out.
Either that or get some cycling shoes with a roomier toe box.

Your shoes are too small and/or you have too thick a sock packed in there. Try removing the insole from the shoe and wearing the thinnest sock you have and see if that alleviates (or ameliorates) the problem.

When shopping for new shoes just check the return policy of the store you order them from (or be upfront if you're going to an LBS) and get a few pairs to see what feels best.

I'm going Saturday to look for new shoes and to get some actual bike socks. Hopefully that will help!

Another thing is that your shoes may be on too tight. Another ST'er told me a trick that has worked for me. When you are tying/fastening your shoes (either running or cycling shoes), scrunch your toes up and then tie the shoes as tight as possible. That will give you the room you need, but also make the shoe tight enough for running or cycling.

I sometimes have to readjust the tightness of my cycling shoes about 20 minutes into my ride, but the above trick usually works the first time for me.