It's called Shimmy, and while it's more common in Mollies, other fish, especially livebearers, can also get it. It is often caused by a chill, and since fish are coldblooded, it takes them awhile to get over it. Disease is another common cause, and since you've got spots, that's my first guess.
Guppies are prone to their own disease, Guppy disease. It's called Tetrahymena, and it has symptoms which resemble a cross between ick & TB, so it can be confusing. it tends to occur most often when the fish are kept in water too sodt and/or with too low a pH, and a chill tends to cause the outbreaks.
Most strains are very easy to cure with standard ick medicines, so there's usually little to worry about except for the speed of the disease's progression, which requires rapid treatment. A few rare strains, however, have been known to wipe out entire guppy farms with horrifying swiftness because they don't respond to normal treatment, but instead need a heavy dose of nitrofurazone. Ironically, overdose of that drug is what has led to most asian imported guppies usually looking so terrible, while at the same time it's those fish which bring the disease into the guppy tanks of the petshops.
All in all, I wouldn't panic in your case. Just treat as you would for ick, maybe a little bit heavier on the doseage if the normal dose doesn't work after 3-4 days.
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