The seed being a photoperiod could still have come from a stray pollen particle from some random place on your plant, but i would have to say my bet is that the seed isn't from your plant if it's a photoperiod plant and you've only grown autoflowers.
Now, if you are (or someone around you is) growing a photoperiod somewhere it's possible.
Again, it sounds like it came from elsewhere than your plant.
So here's a bit in depth
Autoflowering is, at this time, considered to be a double recessive genetic trait, where as photoperiod flowering is considered to be a dominant trait. That means any time an autoflower is crossed with a TRUE photoperiod, a photoperiod is produced. HOWEVER if the photoperiod is a fast version or a quick version or whatever, it will already be crossed with an autoflower and it will have one dominant gene (photoperiod flowering) and one recessive gene (autoflowering). Causing the plant to flower more rapidly with (potentially) a bit less bud than the photoperiod parent but a lot more hardy and tolerant. This generation are usually called FV1 or Q1 or something else catchy by the breeder. They are cool and work great. f
Ifa fast version and an autoflower are crossed, there is a 1 in 4 chance (25%) that a plant will be an autoflower. 1 in 4 will be completely photoperiod dominate while the remaining 50% will remain heterozygous (have both the dominant and recessive gene for the photoperiod vs autoflower trait).
This Fast version and autoflower cross will be the first generation with autoflowers as possible offspring. So unless one of the parents carry the autoflowering trait as either an autoflower itself, or by having the auto trait in it as a recessive trait, i do not think that it is probable that this particular photoperiod seed is derived from an unpollinated feminized autoflowering cannabis plant. If this plant truly did come of virgin birth from an autoflowr, then that plant is a unicorn which should be genetically sequenced asap (i do not mean that rudely, i mean that seriously, i truly believe anything is possible and the best strains have come from almost mythical origin)