Is an illegal forward p an automatic loss of down?

Giants at Minn. Favre had a 1st and 10 at the giant 15, after being flagged for an illegal forward p the next play was 2nd and 13 from the 18. Many replays of the first play were shown and no doubt the network missed something. I hit rewind twice and im sure that was the down and distance of two connsecutive plays and i cant figure out any possible ways this can be.