VYW. I'd check this out with vet. the beginning of summer, with longer period of sunlight and reaching 6 months of age is the typical timeframe for first heat for a cat.
However, if she is already in heat (probably), spaying will have to wait. However, consult your vet on that. The recommendation is to spay or neuter before first heat (4-6 months).
"All female cats (or queens) that have not been spayed will come in heat (or estrus) at stages in their lives. This may happen from as young as the age of 4 months and recur seasonally (usually summer seasons with more daylight). The cat stays in heat for normally 4-6 days and during the summer season will come in heat again every 2-3 weeks. These periods may vary widely. The cat will come out of heat and the cycles will end once the cat has mated or the season ends. After giving birth a female cat may come in heat again in as soon as 7 days although this usually happens after about 4 weeks.
How to tell if your cat is in heat
It is fairly easy to tell when your female cat is in estrus, because all cats in heat display similar symptoms.
The first sign is that the cat will become overly affectionate and rub her rear and even head or neck against all objects, other cats and her owners.
The easiest and most obvious way to tell when your cat is in heat, is the call. Your cat will produce a very different, loud long howl. This is to let other male cats in the area know that she is in estrus and her calling will continue until she mates.
A cat in heat will also bend her body in a mating position, with her rear raised, tail in the air and tread around in the same place.
She also lifts her tail and sprays urine on objects. This is usually done on vertical objects like walls, doors and furniture rather than on the floor or carpet. This urine usually has a more serious odor than the odor left when urinating normally. Cat urine produced when in heat contains more estrogen, and this extreme cat urine smell lets male cats know your cat is in heat.
If your female cat shows most of these above symptoms, the chances are almost 100% that she is in heat!
Male cats that notice a female cat in heat will also change their behavior. They will become more restless and also spray their urine on objects. They may also start calling and howling, becoming more aggressive and territorial."