well, it could have been from the graham crackers, did you do anything with them or just toss them into the beer or boil them or what?
if there was a film, then it was most likely an infection. i'd definitely be concerned about bottle bombs if you got the gusher after 4 days in the bottle. gushers are another sign of infection. if they don't taste good, personally, i'd dump it, but others may try to salvage a few.