A cat raising her hindquarters and swishing her tail can be a sign she is in heat, according to Catster. The article says this posture, called "presenting," signals to males that a female is receptive to mating.
Catster says unspayed cats usually have their first heat between six and nine months old, though some start earlier. After that, a cat typically comes into heat about every 21 days. During months with longer daylight hours, the cycle can repeat every two to three weeks.
The article describes other possible signs of heat. A cat in heat may become more vocal, roll around more often, seek extra attention, spray urine, or try to get outside. Catster notes that none of these signs confirms heat on its own. Spayed females and male cats can also raise their hindquarters, and other causes can explain frequent vocalizing or litter box changes. Catster advises a vet check if a cat shows additional signs that something is wrong.
Catster says an unspayed female that gets outside can become pregnant and faces risks such as traffic and animal attacks. It advises spaying or neutering most cats by 6 months.