Why are my female guppies suddenly frightened?

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It's been about 2 1/2 months now and my female guppies (which were always out and upfront, literally 'swimming pigs' as I call them, because they are 10 times larger than my male guppies, all of which are siblings to one another and are divided in a 30 gallon tank) have now become extremely frightened anytime we approach the tank. One day they were fine, the next completely afraid. It just doesnt make sense. They will all be exactly a year in May.

Any thoughts as to why this would have suddenly happened? We have not added any new fish to the tank. Nothing has changed.

Please advise.
Thanks