I missed my depo-prevera appointment by 3 weeks and am starting to show certain symptoms of pregnancy. is there a chance i may be pregnant?

Low probability: Depo hangs in there for a long time. There is the chance that by interrupting your injection rhythm of every-3-month shots, you could have ovulated; but I highly doubt it. The opposite is also true - you may want to get pregnant & stop Depo, but not ovulate for 9 months! Take a pregnancy test to make sure, and restart Depo shots.