I always wear a full size bigger than my street shoe (sometimes more).  But the number isn't what counts.  Size your shoes so there is a thumbnail width between your longest toe and the front of the shoe.  Measure it standing, not sitting.

What happened is unusual (though it happened to me once in the past that I remember).  Your foot may have slipped in your sock, then the sock bunched up.  Hopefully you're not wearing cotton socks, which are more likely to do this.  Or the insole may have slipped forward slightly in the shoe, then bunched slightly under your foot.  If you have aftermarket insoles, they may have been trimmed short, allowing them to move.

There are a variety of ways to lace your shoes to cure various problems (links below).  Lacing the front (toward the toe) a little tighter can help keep your feet from slipping forward.  This has to be done with a little caution so they're not too tight.

Len

http://www.dickssportinggoods.com/info/index.jsp?categoryId=690482

http://www.fieggen.com/shoelace/lacingmethods.htm

http://www.ehow.com/how_4760788_methods-lacing-nike-shoes.html

http://www.ehow.com/how_4760788_methods-lacing-nike-shoes.html

I'm having the same problem at this moment with only my left running shoe. Usually at mile 2 the arch starts to burn....

I was told to try a new knot- The butterfly knot. Tonight will be the first try with the butterfly knot.  Youtube has the instruction if your interested.