What are the chances of me getting pregnant from a spermicidal condom that fell off during intercourse with just a tiny bit of pre-cum?

Precum has sperm: I think you may be at a little bit of a risk. But, not every situation like this ends in pregnancy. If your period is on time and of a normal length than you probably did not get pregnant. There is also the option of doing a home pregnancy test if you are really worried.

It may be possible.: Most studies have shown low levels of sperm or no sperm in pre-ejaculate with sperm not being motile (however, don't know how promptly samples were examined). One small study published in 2011 showed that there are some individuals with a high motile sperm count in pre-ejaculate. See: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3564677/