I see why you'd say it was a lyretail, because of it's colours only on the bottom and top of the tail, I have one with a similar issue... though I've never seen a female lyretail I cannot say if they are or are not... as for your guppy's pregnancy, if it's her first time she looks pretty far along. I recently had a female guppy that wasn'teven all that fat, which had 20 babies (all of which lived) it's tough to tell on the younger ones, but after they've had a few sets of fry you'll be able to tell right away, their stomack looks huge and baloon like, like they're gonna pop!
Here's a picture of my female that has the same kind of tail as yours (I named her "BabyMaker" with high hopes for her producing many offspring, as guppies do... her first batch, about a week ago were all still born [or something cause they all looked like little yellow eggs... maybe just undeveloped] I knew it, I jinxed it!)