The question implies that your cat has not hissed at you before and therefore the relationship is good. Despite this, at the time that she hissed at you, she perceived you as a predator. A threatening cat faced with a predator produces a sound that is almost identical to that of an angry snake which is intended to cause the predator to hesitate long enough for the cat to escape.
Therefore, you have to ask why your cat perceived you as a predator. It was a temporary state of affairs, I presume. One obvious answer is that your cat felt pain. Perhaps you tried to pet her or pick her up which set off the pain. Therefore at that moment she equated you with a predator causing pain so she hissed.
The first box to tick is to see a veterinarian to check your cat for any causes of pain such as arthritis or an internal injury or painiful condition.
Another possibility is that your cat was excited at the time that she hissed because she had previously been in a hostile situation which she had carried forward in her interactions with you. This is called redirected aggression. She may have instinctively transferred the aggression that she felt towards another animal to you. This would be because she could not express that aggression towards the other animal. This animal might be a stranger cat outside the home in her territory if she is an indoor cat.
A further possibility is that she hissed at you when she was feeding on prey that she had caught or when eating commercial cat food. Sometimes domestic cats who are less than perfectly socialised and perhaps in a slightly excited state can allow themselves to hiss (or growl) at their human caretaker because they feel that the person is going to take their food from them. This is more likely to occur when they bring in prey and the person tries to take it off them. It is a natural instinct to defend their food.
Occasionally, if a cat owner pets and strokes a cat too much it no longer becomes pleasurable for a cat but induces aggression because they perceive the petting as an act of aggression. The hiss is intended to deter the aggression.
Touching on socialisation again, if your cat is only partially socialised she will tend to hiss because living with humans causes anxiety which can set off defensive aggressive behaviour, part of which is hissing.
If you have introduced a new cat or dog into the household which has agitated your resident cat, she may hiss at you because she is generally wound up and in a defensive aggressive mood. It is not directed at you per se but at the situation she finds herself in.
Finally, at the beginning of this article I made the presumption that the relationship between you and your cat was a good one. If, however, it is not, your cat may be slightly anxious permanently making her defensive which could set off an act of defensive aggression including hissing.
These are some of the possible reasons why your cat is hissing at you. Some of them may be obviously correct because the descriptions fit in with what is happening. If not, it’s important that you check that your cat is not in pain which means taking her to a veterinarian as soon as possible.
This is not necessarily a complete list. A good vet, especially a feline specialist, will be able to provide further advice.