My unspayed female cat started rolling around on the floor, arching her back and meowing and purring loudly. Although she does not appear to be sick, she sure sounds like she is in agony. Could my cat have colic, or could she be in heat? I have never seen this before.

Watch for characteristic behaviors.

It sounds like your cat is in heat. Cats with abdominal distress or colic rarely roll and purr: they are more likely to be less active and quiet, even reclusive and go into hiding. Cats with disease of the central nervous system or certain metabolic disorders may display behavioral changes and behave neurotically, as might older cats with excessive thyroid activity (hyperthyroidism). Most hyperthyroid cats will be losing weight in spite of a normal to increased appetite, concurrent with any hyperactivity or excessive vocalization.

The best way to tell if a cat is in heat is to look for behavioral changes. These changes occur in two different phases, depending on the heat cycle. When the female is experiencing proestrus, or the period just before estrus occurs, typically she will become increasingly affectionate, and will rub her face or head against things, tread with the rear feet, and vocalize. Although during proestrus the female cat will show decreasing hostility towards a male cat, she will not allow mounting. These signs may last for one to two days.

The estrus part of the cycle is characterized by increased vocalization, holding the tail to one side, and rolling around on the floor. The cat will exhibit lordosis, or arching or extending the back with the hind limbs raised. Females will often rub along the floor and meow constantly. Some cats may exhibit usual amounts of friendliness to their owners during estrus, although some may become uncharacteristically hostile. Indoor cats may try to escape outside of the house. During estrus, the female is receptive to mating with a male cat. This period may last for six days.

If you do not plan on breeding your cat, I would recommend that you have her spayed as soon as possible. Your veterinarian will be able to determine whether she is actually in heat and the best time to spay her.