If Tabby surprised you with a litter of kittens, you may find they don't look much alike. It's not at all unusual when mom has a litter of babies that are different colors and patterns. If your cat's kittens don't have many similarities, chances are she had multiple partners.
The Heat Cycle of Female Cats
Cats have a heat cycle that tends to occur during the time of year when the days are longer, usually associated with warmer weather. The heat cycle is the time when the cat can actually get pregnant. In some cats, this time can be preceded by a couple of days where she acts a little strangely. She may rub against objects, roll around on the floor and be very vocal. If Tabby went through this, it wasn't because she was in pain -- she was just hormonal.
The heat cycle usually lasts 5 to 8 days, but it can range from 3 to 20 days. When they have completed a cycle, there is usually an average of 10 days before they start another heat cycle (the range is 3 to 14 days). Female cats will have a cycle every 12 to 20 days during breeding season. If Tabby has not been spayed, she will go through these cycles during breeding season and experience hormonal changes until she mates.
Instinct tells Tabby to look for a mate, and if she's outside, she'll look for places that have been urine marked by the male of her choice. She will flirt in her way to signal her intention to the male and they will mate, which can take less than a minute. After breeding, she will take some time to clean herself and then start the process over again.
Multiple Eggs, Multiple Toms
A cat in heat can have multiple eggs, and they can be released over several days of her cycle. If Tabby has chosen more than one Tom during her heat cycle, it's possible she'll be carrying kittens with multiple fathers. Pregnant or not, she will continue to mate until the heat cycle is over. If she is pregnant when the cycle is over, she will not go into heat until after she gives birth.
If You've Already Chosen Tabby's Mate
If you are trying to breed Tabby with a specific male, then she should be confined with him for 24 hours to 4 days. Make sure you keep her confined until her heat cycle is complete to ensure she doesn't seek out an additional partner on her own. However, if you don't want Tabby to be a mother at all, the best thing to do is to have her spayed.